Mayank Mayank - 19 days ago 5
C++ Question

C++ std::function operator=

I am unable to compile a simple program in C++11.
You can have a look at it here http://cpp.sh/9muxf.

#include <functional>
#include <iostream>
#include <exception>
#include <tuple>
#include <map>

using namespace std;

typedef int Json;
typedef string String;

class Integer/*: public PluginHelper*/
{
public:
Json display(const Json& in)
{
cout << "bad" << endl;
return Json();
}

map<String, function<Json(Json)>>* getSymbolMap()
{
static map<String, function<Json(Json)>> x;
auto f = bind(&Integer::display, this);
x["display"] = f;
return nullptr;
}
};


The issue is coming at line
x["display"] = f;


You be of great help if you make me understand what is happening here :).
Can
std::function
not be copied?

Answer

Your problem lies here:

auto f = bind(&Integer::display, this);

Integer::display takes Json const& and you bind it with no explicit arguments. My gcc rejects such a bind expression, but both cpp.sh's compiler and my clang let this compile, possibly incorrectly because the language standard states that:

*INVOKE* (fd, w1, w2, ..., wN) [func.require] shall be a valid expression for some values w1, w2, ..., wN, where N == sizeof...(bound_args)

You can fix your problem by making your bound function object f proper - just add a placeholder for the Json argument:

auto f = bind(&Integer::display, this, placeholders::_1);

demo