定义string类型 编译器还是报错

今天遇到一个有趣的代码

#include<iostream>
#include<map>
//#include<string.h>
using std::cout;
using std::string;
using std::endl;
using std::pair;
int main()
{   string A;//A
   pair <string,string> my_name("yi","dong");
cout<<"my name is "<<my_name.first<<" ";
cout<<my_name.second<<"."<<endl;
return 0;
}

当去掉using std::string换成#include<string.h>时编译器报错了。

the value of ‘string’ is not usable in a constant expression

注释其他代码，main仅剩string A时编译器也出错了

#include <string>

将string库包含到当前编译单元中.

using std::string;
#include<iostream>
using namespace std;
int main()
{
string test("test");
cout<<test<<endl;
return 0;
}

编译这个代码，编译器没有出错
当我们改成下面这个代码,也就是没有引用string

#include<iostream>
using std::cout;
using std::endl;
int main()
{
string test("test");
cout<<test<<endl;
return 0;
}编译器提示如下

lishun@lishun-P43C-A7:~$ g++ 44.cpp -std=c++11 -o p
44.cpp: In function ‘int main()’:
44.cpp:6:5: error: ‘string’ was not declared in this scope
string test("test");
^
44.cpp:6:5: note: suggested alternatives:
In file included from /usr/include/c++/5/iosfwd:39:0,
from /usr/include/c++/5/ios:38,
from /usr/include/c++/5/ostream:38,
from /usr/include/c++/5/iostream:39,
from 44.cpp:1:
/usr/include/c++/5/bits/stringfwd.h:74:33: note:   ‘std::__cxx11::string’
typedef basic_string<char>    string;
^
/usr/include/c++/5/bits/stringfwd.h:74:33: note:   ‘std::__cxx11::string’
44.cpp:7:11: error: ‘test’ was not declared in this scope
cout<<test<<endl;
^
lishun@lishun-P43C-A7:~$

当我加上using std::string;

#include<iostream>
using std::cout;
using std::endl;
using std::string;
int main()
{
string test("test");
cout<<test<<endl;
return 0;
}

编译器能通过

再看看这个代码

#include<iostream>
#include<string>
using std::cout;
using std::endl;
int main()
{
string s("c++");
cout<<s<<endl;
return 0;
}编译器也没有通过

编译单元包含了string

必须使用string才能通过编译器

使用就是string样式 std::string;或者using namespace std;或者 using std::string;

只是中间的引用string类型较为隐蔽，不容易发现。不用using namespace std时才容易出错