I have this function:
template <typename... Args>
void f(Args... args, int last)
{
}
Template deduction fails if I call it without explicit template parameters:
f(2, 2); // candidate expects 1 argument, 2 provided
But giving the explicit template parameters for the parameter pack works:
f<int>(2, 2); // compiles fine
Even though logically speaking, the compiler should be able to deduce that the parameter pack consists of all but the last argument types. How would I fix this?
[temp.deduct.type]/p5:
The non-deduced contexts are:
- [...]
- A function parameter pack that does not occur at the end of the parameter-declaration-list.
To get deduction, you'll have to do
template <typename... Args>
void f(Args... args)
{
}
and slice off the last argument in the body, or make last first instead:
template <typename... Args>
void f(int first, Args... args)
{
}
It's hard to give more specific advice since we don't know what this function template is supposed to do.
From [temp.deduct.type]:
The non-deduced contexts are:
[...] — A function parameter pack that does not occur at the end of the parameter-declaration-list.
Your parameter pack, args, is a non-deduced context since it's not the last parameter. Non-deduced contexts, like the name suggests, cannot be deduced - which leads to a template deduction failure.
When you explicitly provide the template parameters (via f<int>(2, 2)), no deduction has to take place, so the code is fine, you are simply explicitly calling f<int>(int, int).
