Why the MVS compiler can convert argument 'myStruct' to 'myStruct &'. And did not file error C2664: cannot convert 'myStruct' to 'myStruct &'

问题

I have a class where I use std::mem_fn to choose between helper functions.

Why my code get compiled and run If I am missing & in m_funcContainer deceleration ? In the code & commented out with /**/ myStruct/*&*/

std::map < std::string, std::function<void(const myClass*, myStruct/*&*/) >> m_funcContainer

(but in case of m_funcContainerInt the compiler rise compile error)

error C2664: 'void (int &) const' : cannot convert argument 1 from 'int' to 'int &'

I feel that I have not formulate the title of my question in a best way, can you please help me to formulate technically more correct title?

Why the compiler can convert argument 'myStruct' to 'myStruct &' in std::function

My simplified code is

myClass.h

#include <memory>
#include <map>
#include <functional>

struct ExtraFlag
{
};

struct Flag
{
};

struct myStruct
{
    std::shared_ptr<ExtraFlag>         extraFlag;
    std::shared_ptr<Flag>              flag;

    explicit myStruct()
    {
    }
};

class myClass
{
private:
    std::map < std::string, std::function<void(const myClass*, myStruct/*&*/) >> m_funcContainer;
    std::map < std::string, std::function<void(const myClass*, int/*&*/) >> m_funcContainerInt;

private:
    void funcMyStruct(myStruct& arg1) const;
    void funcInt(int& arg1) const;
public:
    myClass();
};

myClass.cpp

#include "myClass.h"
myClass::myClass()
{
    m_funcContainer["func"] = std::mem_fn(&myClass::funcMyStruct);
    myStruct myStructInstance;
    m_funcContainer.at("func")(this, myStructInstance);

    int a;
    m_funcContainerInt["func"] = std::mem_fn(&myClass::funcInt);
    m_funcContainerInt.at("func")(this, a);
}
void myClass::funcMyStruct(myStruct& arg1) const
{}

void myClass::funcInt(int& arg1) const
{}

EDITED I am compiling on Microsoft visual studio 2013

回答1:

Your problem is that MSVC2013 is not a C++ compiler under its default settings. It is compiles a language closely related to C++, but with "extensions". You are being bitten by one of them.

/Za will turn off (most?) language extensions, I believe including the one causing you a problem here.

I have heard reports that some headers that ship with MSVC (system headers) can have problems with /Za. And, code that was compiled and tested with /Za off could have unexpected behavior changes with /Za turned on. I would include it by default in new files or projects, and if you have an old project activate it and test that it doesn't cause problems.

来源：https://stackoverflow.com/questions/33345509/why-the-mvs-compiler-can-convert-argument-mystruct-to-mystruct-and-did-no

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