C++ Lambda with template

Question

I'd like to call a lambda with const template value, in example below I should write lambdaTemplate correctly

#include <iostream> using namespace std; template<int bar, int baz> int foo(const int depth) { return bar + baz + depth; } int main() { ////// call function cout << foo<1, 2>(10) << endl; // 13 ///// call lambda without template value const auto lambda1 = [&](const int v) { return foo<1, 2>(v); }; cout << lambda1(10) << endl; // 13 ///// call lambda with template value const auto lambdaTemplate = [&]<A,B ??? >(const int v) { // <-- ERROR doesn't compile return foo<A,B>(v); }; cout << lambdaTemplate<1,2>(10) << endl; return 0; }

Are you using a compiler that does support template parameters for lambdas? This is a pretty new feature (C++20). In that case the template parameters should be similar to those of foo. — fabian
– fabian, Commented Jan 10, 2021 at 11:56
Even with C++20, the template argument must be deducible (or the nice syntax is not available). Either way, if what you want is a capturing lambdaTemplate<1,2>(10), that's a non-starter. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Jan 10, 2021 at 12:18
You can get lambdaTemplate(constant<1>, constant<2>)(10) to work, however. — Yakk - Adam Nevraumont
– Yakk - Adam Nevraumont, Commented Jan 10, 2021 at 15:12

dfrib · Accepted Answer · 2021-01-12 09:15:10Z

9

You can define lambda like this :

auto lambdaTemplate = [&]<int T1, int T2>(const int v) { return foo<T1, T2>(v); };

and call it like

std::cout << lambdaTemplate.operator()<1,2>(10);

edited Jan 12, 2021 at 9:15

dfrib

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answered Jan 10, 2021 at 12:16

The Philomath

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3 Comments

dfrib Over a year ago

Note that lambdaTemplate is not a dependent name, and you shall thus not use the template keyword when accessing the conversion function template. At best this is redundant, but at worst ill-formed (but accepted by some compilers nonetheless), as covered in the following Q&A.

Davis Herring Over a year ago

@dfrib: The parser guides are nowadays allowed in non-dependent contexts (which I fear merely encourages the popular confusion about their purpose), so long as it is in fact a template-id that follows.

dfrib Over a year ago

@DavisHerring I see, thanks. I see I even pointed this out in my self-answer to the linked Q&A, but had since forgot it; it is indeed a bit confusing (and somewhat counter-intuitive) that this is allowed.

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