c – std :: make_pair：无法将’ch'(类型’char’)转换为’char \u0026\u0026′

参见英文答案 > C++11 make_pair with specified template parameters doesn’t compile1个
以下代码有什么问题：

#include <ctime>
#include <vector>
#include <utility>
#include <algorithm>
#include <iostream>

int main()
{
    std::vector< std::pair< char,unsigned > > vec;

    for( unsigned i = 0; i < 100; ++i )
    {
        char ch = 0;
        unsigned number = 0;

        do {
            ch = i;
            number = i;

        } while( std::find( vec.begin(),vec.end(),std::make_pair< char,unsigned >( ch,number ) ) != vec.end() );

        std::cout << ch << number << '\n';

        vec.push_back( std::make_pair< char,number ) );
    }
}

它可以很好地编译：

g++ test.cxx

但失败了：

$g++ -std=c++11 test.cxx                                                                                                              /tmp
test.cxx: In function 'int main()':
test.cxx:21:98: error: no matching function for call to 'make_pair(char&,unsigned int&)'
test.cxx:21:98: note: candidate is:
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:65:0,from /usr/include/c++/4.7/vector:61,from test.cxx:3:
/usr/include/c++/4.7/bits/stl_pair.h:268:5: note: template<class _T1,class _T2> constexpr std::pair<typename std::__decay_and_strip<_Tp>::__type,typename std::__decay_and_strip<_T2>::__type> std::make_pair(_T1&&,_T2&&)
/usr/include/c++/4.7/bits/stl_pair.h:268:5: note:   template argument deduction/substitution Failed:
test.cxx:21:98: note:   cannot convert 'ch' (type 'char') to type 'char&&'
test.cxx:25:69: error: no matching function for call to 'make_pair(char&,unsigned int&)'
test.cxx:25:69: note: candidate is:
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:65:0,_T2&&)
/usr/include/c++/4.7/bits/stl_pair.h:268:5: note:   template argument deduction/substitution Failed:
test.cxx:25:69: note:   cannot convert 'ch' (type 'char') to type 'char&&'

解决方法

解：

而不是以这种方式显式指定make_pair<>()的模板参数：

std::make_pair< char,number )

只是让他们推断：

std::make_pair( ch,number )

说明：

这个指南背后的基本原理可以通过std :: make_pair<>()的定义方式找到,并且模板参数推导的方式适用于通用引用.根据C 11标准第20.3.3 / 8-9段：

template <class T1,class T2>
pair<V1,V2> make_pair(T1&& x,T2&& y);

Returns: pair<V1,V2>(std::forward<T1>(x),std::forward<T2>(y));
where V1 and V2 are determined as follows: Let Ui be decay<Ti>::type for each Ti. Then each Vi is X& if Ui equals reference_wrapper<X>,otherwise Vi is Ui. [ Example: In place of:

return pair<int,double>(5,3.1415926); // explicit types

a C++ program may contain:

return make_pair(5,3.1415926); // types are deduced

—end example ]

这里,T1和T2意在推断.通过自己明确指定模板参数,您将开始使用make_pair<>()的类型推导机制来生成正确的返回类型,从而强制实例化接受对char和rvalue的rvalue引用的函数对无符号的引用：

... make_pair(char&&,unsigned&&)

但是,您没有在输入中提供rvalues,因为ch和number是左值.这就是编译器所抱怨的.

替代方案：

另请注意,您可以隐式构造一个std :: pair对象,这样就可以保存您从调用make_pair<>()的过程中：

vec.push_back( { ch,number } );

c – std :: make_pair：无法将’ch'(类型’char’)转换为’char \u0026\u0026′

解决方法

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