Passing a lambda to a template function taking a std::function as parameter

Question

I have a template function taking a std::function parameter, the template arguments defining function signature:

template<typename... Args>
void addController(const char * name, const std::function<bool(Args...)> & func);

It works passing a std::function variable:

std::function<bool()> foo_func = [this]()->bool {return this->isFoo(); };
addController<>("foo", foo_func);  //Works

But if I pass the lambda directly, it fails deducing the type:

//Fails to compile
addController<>("foo", [this]()->bool {return this->isFoo(); });

And using a non template function works:

void addControllerNoArg(const char * name, std::function<bool()> func);
addControllerNoArg("foo", [this]() {return this->isFoo(); });  //Works

I need the <typename... Args> template for unwrapping a variant vector argument tables into the function call. This actually does work in the implementation, the only issue is I can't pass a lambda to addController directly.

Minimal example: https://onlinegdb.com/MS1cEreKhk

template <typename FunctionType> void addControler(F f);. Don't confuse std::function to be the goto type whenever you need to pass around a callable. You need it for type erasure, ie when you need one object that can store all kinds of callables — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 28, 2022 at 11:59
I need <typename... Args> because internally, arguments are retrieved in a variant table that needs to be unwrapped into func call — galinette
– galinette, Commented Sep 28, 2022 at 12:04
Added a minimal example but there isn't more inside than what shown above — galinette
– galinette, Commented Sep 28, 2022 at 12:14
#includes and the context for this is a lot, when someone else wants to either reproduce your error or test their answer. — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 28, 2022 at 12:17

Jarod42 · Accepted Answer · 2022-09-28 13:00:34Z

7

Way to go is generic callable:

template<typename F>
void addController(const char* name, F f)

You can forward it to your std::function version if needed:

template <typename... Args>
void addController(const char* name, const std::function<bool(Args...)> & func)
{
   /*..*/
}

template <typename F>
void addController(const char* name, F f)
{
    addController(name, std::function{f}); // CTAD used, so C++17 required
}

edited Sep 28, 2022 at 13:00

answered Sep 28, 2022 at 12:08

Jarod42

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4 Comments

NathanOliver Over a year ago

Yeah overloads :)

galinette Over a year ago

I've tested this, but it says missing template arguments before {f} onlinegdb.com/3dSMUCSYvG

NathanOliver Over a year ago

@galinette You need to compile with C++17 or newer.

Yksisarvinen Over a year ago

@galinette That compiler doesn't seem to enable C++17 by default. It works here: godbolt.org/z/Y6G43fd9n

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