CODEWITHSUNDEEP
c++templates
CS Pei
CS Pei

Reputation: 11047

Pass a function template as a parameter in C++

For example, I want to get a list of maximum values from two sequences, left and right, and save the results in max_seq, which are all previously defined and allocated,

std::transform(left.begin(), left.end(), right.begin(), max_seq.begin(), &max<int>);

But this won't compile because the compiler says

 note:   template argument deduction/substitution failed

I know I can wrapper "std::max" inside a struct or inside a lambda. But is there a way directly use std::max without wrappers?

Upvotes: 5

Views: 701

Answers (2)

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

Template expansion and instanciation occur at compile time. So you can pass a template function only to templates.

You could pass an instanciated (templated) function at run-time (then it is an "ordinary" C++ function).

Upvotes: 0

Praetorian
Praetorian

Reputation: 109089

std::max has multiple overloads, so the compiler is unable to determine which one you want to call. Use static_cast to disambiguate and your code will compile.

static_cast<int const&(*)(int const&, int const&)>(std::max)

You should just use a lambda instead

[](int a, int b){ return std::max(a, b); }

Live demo

Upvotes: 7

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