How can I pass a function using default parameter(s) to std:: thread?

Question

So I have gcc version 4.8.1, g++ version 4.6.4, using flags: -std=c++0x and -pthread.

I simplified my problem down to the code shown and still get the original error.

What I have below compiles, but when I uncomment the two lines for thread "two", I get the error message shown below the code

#include <iostream>
#include <thread>
using namespace std;

void print_int(int x=7);
void print_A(){
    cout << "A\n";
}

int main(){
    thread one (print_int,17);
    //thread two (print_int);
    thread three (print_A);

    one.join();
    //two.join();
    three.join();

    return 0;
}

void print_int(int x){
    cout << x << '\n';
}

I tried to parse through the error messages but I still have no clue what's going on...

    In file included from /usr/include/c++/4.6/thread:39:0,
             from def_params.cpp:2:

    /usr/include/c++/4.6/functional: In member function ‘void std::_Bind_result<_Result, _Functor(_Bound_args ...)>::__call(std::tuple<_Args ...>&&, std::_Index_tuple<_Indexes ...>, typename std::_Bind_result<_Result, _Functor(_Bound_args ...)>::__enable_if_void<_Res>::type) [with _Res = void, _Args = {}, int ..._Indexes = {}, _Result = void, _Functor = void (*)(int), _Bound_args = {}, typename std::_Bind_result<_Result, _Functor(_Bound_args ...)>::__enable_if_void<_Res>::type = int]’:

    /usr/include/c++/4.6/functional:1378:24:   instantiated from ‘std::_Bind_result<_Result, _Functor(_Bound_args ...)>::result_type std::_Bind_result<_Result, _Functor(_Bound_args ...)>::operator()(_Args&& ...) [with _Args = {}, _Result = void, _Functor = void (*)(int), _Bound_args = {}, std::_Bind_result<_Result, _Functor(_Bound_args ...)>::result_type = void]’

    /usr/include/c++/4.6/thread:117:13:   instantiated from ‘void std::thread::_Impl<_Callable>::_M_run() [with _Callable = std::_Bind_result<void, void (*())(int)>]’

    def_params.cpp:26:9:   instantiated from here

    /usr/include/c++/4.6/functional:1287:4: error: too few arguments to function

Any suggestions or solutions? Thanks in advance!

Answer 1

thread two (print_int);

You can't do this, default arguments in C++ are only used at the call site (e.g. when you call print_int() yourself) as a syntactic shortcut, but aren't used when e.g. std::thread invokes the same function passed to it as an argument. The problem is std::thread gets given a pointer to a one-argument function, but is not passed an argument to go with it, so it doesn't know how to call it, and there's no way to look up a function's default argument given a pointer to the function.

The simplest solutin is to use a lambda to call the function, as that call can use the default argument:

thread two( []{ print_int(); } );