The following code does not compile because the compiler deduce a template parameter to be int
while it would be required to be int &
. See it on Coliru here.
#include <iostream>
#include <utility>
template <class F, class... ArgsType>
void do_something(F f, ArgsType... args)
{
f(std::forward<ArgsType>(args)...);
}
int main()
{
int s = 2;
int i = 1;
auto add = [](const int a, int& sum) { sum += a; };
do_something(add, i, s);
std::cout << s << std::endl;
return 0;
}
Error:
main.cpp: In instantiation of 'void do_something(F, ArgsType ...) [with F = main()::<lambda(int, int&)>; ArgsType = {int, int}]':
main.cpp:15:27: required from here
main.cpp:7:6: error: no match for call to '(main()::<lambda(int, int&)>) (int, int)'
f(std::forward<ArgsType>(args)...);
~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:7:6: note: candidate: 'void (*)(int, int&)' <conversion>
main.cpp:7:6: note: conversion of argument 3 would be ill-formed:
main.cpp:7:6: error: cannot bind non-const lvalue reference of type 'int&' to an rvalue of type 'int'
main.cpp:14:40: note: candidate: 'main()::<lambda(int, int&)>' <near match>
auto add = [](const int a, int& sum) { sum += a; };
^
main.cpp:14:40: note: conversion of argument 2 would be ill-formed:
main.cpp:7:6: error: cannot bind non-const lvalue reference of type 'int&' to an rvalue of type 'int'
f(std::forward<ArgsType>(args)...);
~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ideally, the third argument of do_something
should be deduced as int&
. One way to do this is to explicitly pass the template parameters as
#include <iostream>
#include <utility>
template <class F, class... ArgsType>
void do_something(F f, ArgsType... args)
{
f(std::forward<ArgsType>(args)...);
}
int main()
{
int s = 2;
int i = 1;
auto add = [](const int a, int& sum) { sum += a; };
do_something<decltype(add), const int, int&>(add, i, s);
std::cout << s << std::endl;
return 0;
}
See it on Coliru here.
While the solution works, I find it inconvenient, because it forces me to provide all template types of do_something
, which is not optimal especially if, say, I have a more complex example with several parameters, or if I would like to insert the lambda function add
directly as a parameter of do_something
:
do_something([](const int a, int& sum) { sum += a; }, i, s);
Is there a more convenient way to force just the third parameter to be deduced as int &
?
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