CODEWITHSUNDEEP
c++11functional-programmingfunction-pointers
James Franco
James Franco

Reputation: 4716

std::function still requires parameters when no place holders are coded in

I wanted to know what I might be doing wrong here. This is my code sample. Its just a rough code sample depicting the use of function pointers.In the following example the fuunction pointer takes in a parameter.However that parameter was already assigned during function pointer assignment.Now in order to invoke the function pointer i still need to provide a parameter value although that value is nver used (unless i used std::placeholder). My question is how do i invoke a function pointer that requires a parameter and whose parameter has already been assigned without passing in a parameter. 

#include <iostream>
#include <functional>

using namespace std;

std::function<std::string(std::string)> fptr;

std::string foo()
{
    return fptr();  //--->statement A - I need to invoke this funct ptr without passing a parameter as a parameter has already been assigned in statement B 
}

std::string bar(std::string p)
{
    return p;
}

int main()
{
  fptr = std::bind(&bar,"Hello");  --->statement B
  std::cout << foo();
}

Notice in std::bind I did not use any placeholders and "Hello" is the parameter to the function bar. My question is why does 

return fptr();

not work. If i do the following

return fptr("Bye");

It works and returns "Hello" . (No point of passing parameter during fptr call) Why does it not work without a parameter ?

Upvotes: 0

Views: 43

Answers (1)

user1887915
user1887915

Reputation: 1299

The result of std::bind allows you passing more arguments than it needed, and ignore these extra arguments. In your case, std::bind(&bar, "Hello") can be called without arguments, or, declared by fptr, with one std::string argument.

The solution to your problem is easy, just change the type of fptr to std::function<std::string()>, which accept no arguments and return a string.

Upvotes: 3

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