C++程序设计（第4版）读书笔记

变量赋值

常用的变量赋值都是用“=”去赋值的

 int i = ;

但是如果把一个浮点数赋值给i的话，就会造成精度损失，在C++中最好使用初始化列表的方式“{}”给变量赋值，这样可以保证不会发生某些可能导致信息丢失的类型转换

 #include <iostream>

 using namespace std;

 int main() {

     int i {2.3};

     return ;

 }

比如这样声明，编译器就会报错

<source>: In function 'int main()':

5 : <source>:5:12: error: narrowing conversion of '2.2999999999999998e+0' from 'double' to 'int' inside { } [-Wnarrowing]

int i {2.3};

Compiler exited with result code 1

auto

通过一个for循环来学习使用auto的方法

 #include <iostream>

 using namespace std;

 int main() {

     int v[] = {, , , , };

     for (auto x : v) {

         cout << x << "\t";

     }

     cout << endl;

     for (auto i = ; i < sizeof(v) / sizeof(int); ++i) {

         cout << v[i] << "\t";

     }

     cout << endl;

     for (auto x : {, , , , }) {

         cout << x << "\t";

     }

     cout << endl;

     for (auto x : v) {

         x += ;

     }

     for (auto x : v) {

         cout << x << "\t";

     }

     cout << endl;

     /* 对于不带引用的情况，可以理解为对于v的每个元素将其从头到尾依次放入x并打印 */

     for (auto & x : v) {

         x += ;

     }

     for (auto x : v) {

         cout << x << "\t";

     }

     cout << endl;

     return ;

 }

运行结果如下：

0	1	2	3	4

0	1	2	3	4

0	1	2	3	4

0	1	2	3	4

1	2	3	4	5

update 2017-11-02：范围for循环对指针无效，因为我们不知道数组的大小了

结构

在结构里面我们先完成一个最简单的vector，再下面介绍的类中我们可以看到一些优化的部分，可以看到类的操作会更加简单，使用者根本就无需关注里面的实现。

先来看结构实现的代码吧

 struct Vector

 {

     int sz;

     double * elem;

 };

 void vector_init(Vector & v, int s)

 {

     v.elem = new double[s];

     v.sz = s;

 }

 double read_and_sum(int s)

 {

     Vector v;

     vector_init(v, s);

     for (int i = ; i != s; ++i) {

         cin >> v.elem[i];

     }

     double sum = ;

     for (int i = ; i != s; ++i) {

         sum += v.elem[i];

     }

     return sum;

 }

 int main()

 {

     cout << read_and_sum() << endl;

     return ;

 }

运行结果：

类

我们先简单的介绍一下类，在后面会不断的完善，这里我们就仅仅简单的引入构造函数、初始化列表、运算符重载的概念而已。

具体比上面结构的优点，我们还是看代码吧：

 #include <iostream>

 #include <new>

 using namespace std;

 class Vector

 {

 public:

     /*

         C++11这样用

         低版本的话就是 Vector(int s) : elem(new double[s]), sz(s) {}

      */

     Vector(int s) : elem{new double[s]}, sz{s}

     {

     }

     double & operator[](int i)

     {

         return elem[i];

     }

     int size()

     {

         return sz;

     }

 private:

     double * elem;

     int sz;

 };

 double read_and_sum(int s)

 {

     Vector v(s);

     for (int i = ; i != v.size(); ++i) {

         cin >> v[i];

     }

     double sum = ;

     for (int i = ; i != v.size(); ++i) {

         sum += v[i];

     }

     return sum;

 }

 int main()

 {

     cout << read_and_sum() << endl;

     return ;

 }

运行结果：

枚举

书上的枚举前面加了class

 #include <iostream>

 #include <new>

 using namespace std;

 enum class Traffic_light

 {

     green,

     yellow,

     red

 };

 Traffic_light & operator++(Traffic_light & t)

 {

     switch(t)

     {

         case Traffic_light::green:

             return t = Traffic_light::yellow;

         case Traffic_light::yellow:

             return t = Traffic_light::red;

         case Traffic_light::red:

             return t = Traffic_light::green;

     }

 }

 int main()

 {

     Traffic_light src = Traffic_light::green;

     for (int i = ; i < ; ++i) {

         Traffic_light next = ++src;

         cout << next << endl;

     }

     return ;

 }

在编译的时候出错了：

main.cpp: In function 'int main()':

main.cpp:33:8: error: no match for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream<char>}' and 'Traffic_light')

   cout << next << endl;

   ~~~~~^~~~~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:108:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(__ostream_type& (*__pf)(__ostream_type&))

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:108:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&) {aka std::basic_ostream<char>& (*)(std::basic_ostream<char>&)}'

/usr/local/include/c++/7.2.0/ostream:117:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>; std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]

       operator<<(__ios_type& (*__pf)(__ios_type&))

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:117:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}'

/usr/local/include/c++/7.2.0/ostream:127:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(ios_base& (*__pf) (ios_base&))

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:127:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'std::ios_base& (*)(std::ios_base&)'

/usr/local/include/c++/7.2.0/ostream:166:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(long __n)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:166:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long int'

/usr/local/include/c++/7.2.0/ostream:170:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(unsigned long __n)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:170:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long unsigned int'

/usr/local/include/c++/7.2.0/ostream:174:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(bool __n)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:174:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'bool'

In file included from /usr/local/include/c++/7.2.0/ostream:693:0,

                 from /usr/local/include/c++/7.2.0/iostream:39,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/bits/ostream.tcc:91:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char; _Traits = std::char_traits<char>]

     basic_ostream<_CharT, _Traits>::

     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/local/include/c++/7.2.0/bits/ostream.tcc:91:5: note:   no known conversion for argument 1 from 'Traffic_light' to 'short int'

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:181:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(unsigned short __n)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:181:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'short unsigned int'

In file included from /usr/local/include/c++/7.2.0/ostream:693:0,

                 from /usr/local/include/c++/7.2.0/iostream:39,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/bits/ostream.tcc:105:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char; _Traits = std::char_traits<char>]

     basic_ostream<_CharT, _Traits>::

     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/local/include/c++/7.2.0/bits/ostream.tcc:105:5: note:   no known conversion for argument 1 from 'Traffic_light' to 'int'

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:192:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(unsigned int __n)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:192:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'unsigned int'

/usr/local/include/c++/7.2.0/ostream:201:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(long long __n)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:201:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long long int'

/usr/local/include/c++/7.2.0/ostream:205:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(unsigned long long __n)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:205:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long long unsigned int'

/usr/local/include/c++/7.2.0/ostream:220:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(double __f)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:220:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'double'

/usr/local/include/c++/7.2.0/ostream:224:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(float __f)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:224:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'float'

/usr/local/include/c++/7.2.0/ostream:232:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(long double __f)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:232:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'long double'

/usr/local/include/c++/7.2.0/ostream:245:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]

       operator<<(const void* __p)

       ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:245:7: note:   no known conversion for argument 1 from 'Traffic_light' to 'const void*'

In file included from /usr/local/include/c++/7.2.0/ostream:693:0,

                 from /usr/local/include/c++/7.2.0/iostream:39,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/bits/ostream.tcc:119:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]

     basic_ostream<_CharT, _Traits>::

     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/local/include/c++/7.2.0/bits/ostream.tcc:119:5: note:   no known conversion for argument 1 from 'Traffic_light' to 'std::basic_ostream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}'

In file included from /usr/local/include/c++/7.2.0/string:52:0,

                 from /usr/local/include/c++/7.2.0/bits/locale_classes.h:40,

                 from /usr/local/include/c++/7.2.0/bits/ios_base.h:41,

                 from /usr/local/include/c++/7.2.0/ios:42,

                 from /usr/local/include/c++/7.2.0/ostream:38,

                 from /usr/local/include/c++/7.2.0/iostream:39,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/bits/basic_string.h:6082:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)

     operator<<(basic_ostream<_CharT, _Traits>& __os,

     ^~~~~~~~

/usr/local/include/c++/7.2.0/bits/basic_string.h:6082:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>' and 'Traffic_light'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/bits/ios_base.h:46:0,

                 from /usr/local/include/c++/7.2.0/ios:42,

                 from /usr/local/include/c++/7.2.0/ostream:38,

                 from /usr/local/include/c++/7.2.0/iostream:39,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/system_error:217:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::error_code&)

     operator<<(basic_ostream<_CharT, _Traits>& __os, const error_code& __e)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/system_error:217:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const std::error_code&'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:497:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT)

     operator<<(basic_ostream<_CharT, _Traits>& __out, _CharT __c)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:497:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   deduced conflicting types for parameter '_CharT' ('char' and 'Traffic_light')

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:502:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char)

     operator<<(basic_ostream<_CharT, _Traits>& __out, char __c)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:502:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'char'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:508:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char)

     operator<<(basic_ostream<char, _Traits>& __out, char __c)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:508:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'char'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:514:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char)

     operator<<(basic_ostream<char, _Traits>& __out, signed char __c)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:514:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'signed char'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:519:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char)

     operator<<(basic_ostream<char, _Traits>& __out, unsigned char __c)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:519:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'unsigned char'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:539:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const _CharT*)

     operator<<(basic_ostream<_CharT, _Traits>& __out, const _CharT* __s)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:539:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   mismatched types 'const _CharT*' and 'Traffic_light'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/ostream:693:0,

                 from /usr/local/include/c++/7.2.0/iostream:39,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/bits/ostream.tcc:321:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*)

     operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/bits/ostream.tcc:321:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const char*'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:556:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*)

     operator<<(basic_ostream<char, _Traits>& __out, const char* __s)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:556:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const char*'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:569:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*)

     operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:569:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const signed char*'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:574:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*)

     operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:574:5: note:   template argument deduction/substitution failed:

main.cpp:33:11: note:   cannot convert 'next' (type 'Traffic_light') to type 'const unsigned char*'

   cout << next << endl;

           ^~~~

In file included from /usr/local/include/c++/7.2.0/iostream:39:0,

                 from main.cpp:1:

/usr/local/include/c++/7.2.0/ostream:682:5: note: candidate: template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&)

     operator<<(_Ostream&& __os, const _Tp& __x)

     ^~~~~~~~

/usr/local/include/c++/7.2.0/ostream:682:5: note:   template argument deduction/substitution failed:

/usr/local/include/c++/7.2.0/ostream: In substitution of 'template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&) [with _Ostream = std::basic_ostream<char>&; _Tp = Traffic_light]':

main.cpp:33:11:   required from here

/usr/local/include/c++/7.2.0/ostream:682:5: error: no type named 'type' in 'struct std::enable_if<false, std::basic_ostream<char>&>'

main.cpp: In function 'Traffic_light& operator++(Traffic_light&)':

main.cpp:25:1: warning: control reaches end of non-void function [-Wreturn-type]

应该是没有重载标准输出导致的，这个后续再完善吧，如果把class去掉，就可以正常运行了。囧……

静态断言 static_assert

static_assert能用于任何可以表达为常量表达式的东西，static_assert(A, S)的作用是当A不为true时把S作为一条编译器错误信息输出，这是C++11的特性。

static_assert最重要的用途是为泛型编程中作为形参的类型设置断言。

 #include <iostream>

 #include <cassert>

 using namespace std;

 int main()

 {

     const int i = ;

     static_assert(i < , "5 is larger then 4");

     return ;

 }

编译结果：

main.cpp: In function 'int main()':

main.cpp:10:9: error: static assertion failed: 5 is larger then 4

         static_assert(i < 4, "5 is larger then 4");

         ^~~~~~~~~~~~~

如果上述代码中的i不为常量，那么编译就直接报错了

main.cpp: In function 'int main()':

main.cpp:10:5: error: non-constant condition for static assertion

     static_assert(i < 4, "5 is larger then 4");

     ^~~~~~~~~~~~~

main.cpp:10:5: error: the value of 'i' is not usable in a constant expression

main.cpp:8:9: note: 'int i' is not const

     int i = 5;

另外可以用static_assert来判断编译的平台，达到尽早检查的目的

 #include <iostream>

 #include <cassert>

 using namespace std;

 int main()

 {

     static_assert(sizeof(void *) == , "64-bit code generation is not supported.");

     const int i = ;

     static_assert(i < , "5 is larger then 4");

     return ;

 }

编译结果：

main.cpp: In function 'int main()':

main.cpp:8:5: error: static assertion failed: 64-bit code generation is not supported.

     static_assert(sizeof(void *) == 4, "64-bit code generation is not supported.");

     ^~~~~~~~~~~~~

main.cpp:12:5: error: static assertion failed: 5 is larger then 4

     static_assert(i < 4, "5 is larger then 4");

     ^~~~~~~~~~~~~

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