How to pass generic lambda into function

Question

I have the following C++ code:

template <class T1, class T2, class T3>
void MetaTypeHandler(T1 lambda1, T2 lambda2, T3 lambda3) {
 lambda1(1);
 lambda2('x');
 lambda3(true);
}

int main() {
  auto f = [] (auto x) {};
  MetaTypeHandler(f,f,f);
}

Passing f multiple times is ugly. Is it possible to write MetaTypeHandler() so that f is passed only 1 or 2 times? I think that template template parameters may help, but can't wrap my head around them.

score 6 · Accepted Answer · answered May 12 '19 at 09:53

6

I don't understand the problem.

It's a generic lambda.

Substantially the object of a struct with a template operator() in it.

So you can pass it one time only and use it with all types you want

#include <iostream>

template <typename T>
void MetaTypeHandler (T lambda)
 {
   lambda(42);
   lambda('x');
   lambda("abc");
   lambda(true);
 }

int main()
 {
   MetaTypeHandler(
      [](auto const & x){ std::cout << "x is " << x << std::endl;});
 }

answered May 12 '19 at 09:53

max66

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"struct with a template operator() in it" - thanks, now I got it. I really should learn C++ once :D – Bulat May 12 '19 at 10:05
@Bulat See https://cppinsights.io/lnk?code=I2luY2x1ZGUgPGlvc3RyZWFtPgoKdGVtcGxhdGUgPHR5cGVuYW1lIFQ+CnZvaWQgTWV0YVR5cGVIYW5kbGVyIChUIGxhbWJkYSkKIHsKICAgbGFtYmRhKDQyKTsKICAgbGFtYmRhKCd4Jyk7CiAgIGxhbWJkYSgiYWJjIik7CiAgIGxhbWJkYSh0cnVlKTsKIH0KCmludCBtYWluKCkKIHsKICAgTWV0YVR5cGVIYW5kbGVyKAogICAgICBbXShhdXRvIGNvbnN0ICYgeCl7IHN0ZDo6Y291dCA8PCAieCBpcyAiIDw8IHggPDwgc3RkOjplbmRsO30pOwogfQ==&insightsOptions=cpp17&std=cpp17&rev=1.0 – JVApen May 12 '19 at 10:07
@JVApen thank you very much! Looks that this site may help me in many situations – Bulat May 12 '19 at 10:12

score 2 · Answer 2 · answered May 12 '19 at 10:07

You can just provide an overload:

#include <iostream>

template <class T1, class T2, class T3>
void MetaTypeHandler(T1 lambda1, T2 lambda2, T3 lambda3) {
 lambda1(1);
 lambda2('x');
 lambda3(true);
}

template <class T>
void MetaTypeHandler(T lambda)
{
    MetaTypeHandler(lambda, lambda, lambda);
}

int main() {
  auto f = [] (auto x) {std::cout << x << std::endl;};
  MetaTypeHandler(f);
}

So you can pass 3 different handlers, or one handler executing 3 times.

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How to pass generic lambda into function

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