没有重载函数的实例 std::unordermap.insert

Question

我声明了一个streamclass，但是在定义它的成员函数的时候，想往private成员里面插入东西的时候总是报错unordered_map，请问如何解决这个？ stream.h

#include <unordered_map>

class stream
{
private:
    std::unordered_map<size_t, std::string> word_count;
public:
    void reassemblestream() const;
};

stream.cpp

#include"stream.h"
using namespace std;
void stream::reassemblestream() const
{
    static  size_t a = 1;
    string fe = "dfi";
    word_count.insert({ 1,"dfs" });
    word_count.insert(make_pair(a, fe));
}

word_count.insert 函数的 stream.cpp 发生错误。这是错误信息：

E1087 There is no overloaded function "std::unordered_map<_Kty, _Ty, _Hasher, _Keyeq, _Alloc>::insert [where _Kty=size_t, _Ty=std::string, _Hasher=std:: hash<size_t>, _Keyeq=std::equal_to<size_t>, _Alloc=std::allocator<std::pair<const size_t, std::string>>]" instance (object contains type qualifiers that prevent matching)

我用的是visual studio2019.

Answer 1

如果使用 clang++ 构建程序，我们会得到更清晰的错误消息：

source>:17:16: error: no matching member function for call to 'insert'
    word_count.insert({ 1,"dfs" });
    ~~~~~~~~~~~^~~~~~
/opt/compiler-explorer/gcc-11.1.0/lib/gcc/x86_64-linux-gnu/11.1.0/../../../../include/c++/11.1.0/bits/unordered_map.h:557:7: note: candidate function not viable: 'this' argument has type 'const std::unordered_map<size_t, std::string>' (aka 'const unordered_map<unsigned long, basic_string<char>>'), but method is not marked const
      insert(value_type&& __x)

只需去掉const限定符，因为你是通过调用insert函数来修改数据成员，函数reassemblestream不能被标记为const(相关question 对于 const 成员函数):

#include <unordered_map>
#include <string>

class stream
{
private:
    std::unordered_map<size_t, std::string> word_count;
public:
    void reassemblestream();
};


void stream::reassemblestream() 
{
    static  size_t a = 1;
    std::string fe = "dfi";
    word_count.insert({ 1,"dfs" });
    word_count.insert(make_pair(a, fe));
}

没有重载函数的实例 std::unordermap.insert

no instance of overloaded function std::unordermap.insert

c++

stl

c++11