auto f1 = []<typename T>(T) { return T{}; };
auto f2 = []<typename T>() { return T{}; };
int main()
{
f1(1); // ok
f2<int>(); // err: expected primary-expression before 'int'
}
Why does C++20 not allow to call a generic lambda with an explicit type?
A lambda is also just a function object, so you need to have a () to call it, there is no way around it (except of course some function that invokes the lambda like std::invoke ). If you want you can drop the () after the capture list, because your lambda doesn't take any parameters.
Generic lambdas were introduced in C++14 . Simply, the closure type defined by the lambda expression will have a templated call operator rather than the regular, non-template call operator of C++11 's lambdas (of course, when auto appears at least once in the parameter list).
Lambdas support with C++20 template parameters, can be default-constructed and support copy-assignment, when they have no state, and can be used in unevaluated contexts. Additionally, they detect when you implicitly copy the this pointer. This means a significant cause of undefined behavior with lambdas is gone.
Significance of Lambda Function in C/C++ Lambda Function − Lambda are functions is an inline function that doesn't require any implementation outside the scope of the main program. Lambda Functions can also be used as a value by the variable to store.
The correct syntax to invoke overloaded template operator () function supplying template parameters is
auto f1 = []<typename T>(T) { return T{}; };
auto f2 = []<typename T>() { return T{}; };
int main()
{
f1(1); // ok
f2.operator ()<int>(); // Ok
}
https://godbolt.org/z/YzY7njPGh
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