Cannot initialize a lambda member variable with a default argument

Question

I would like to compile the code below with c++17, so that I can pass any function (lambda) that has a specific signature, int(int), while also allowing the default argument:

template <class F = int(int)> // for deduction
struct A{
        A(F f = [] (int x){return x;}) : f_{f} {}

        F f_;
};

int main() {
        A work([](int x){return x + 1;});
        A not_work; // compile error.
}

However, clang emits an error:

a.cpp:6:4: error: data member instantiated with function type 'int (int)'
        F f_;
          ^
a.cpp:11:4: note: in instantiation of template class 'A<int (int)>' requested here
        A not_work;
          ^

I don't understand why the member f_ can be initialized when I pass the lambda and the default lambda argument cannot be?

Meanwhile, is there any better way to do this?

songyuanyao · Accepted Answer · 2021-12-01 03:10:22Z

3

As the error message said, you can't declare a data member with a function type like int(int).

When passing a lambda to the constructor, the template parameter F would be deduced as the lambda closure type by CTAD (since C++17); when passing nothing F will use the default argument int(int) and the data member f_'s type would be int(int) too, which causes the error.

You might use a function pointer type (lambdas without capture could convert to function pointer implicitly) or std::function<int(int)>. E.g.

template <class F = int(*)(int)> // for deduction
struct A{
        A(F f = [] (int x){return x;}) : f_{f} {}

        F f_;
};

Or

template <class F = std::function<int(int)>> // for deduction
struct A{
        A(F f = [] (int x){return x;}) : f_{f} {}

        F f_;
};

edited Dec 1, 2021 at 3:10

answered Dec 1, 2021 at 2:39

songyuanyao

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2 Comments

wk_j Over a year ago

Thanks, but I have another question. Then why the line A work([](int x){return x + 1;}); works? It seems strange to me that work and not_work object both take lambda as an argument in their ctor, while only one of them works.

Miles Budnek Over a year ago

@wk_j Because when you pass a lambda to A's constructor, F is deduced to be the anonymous lambda type rather than int(int), but when you supply no parameter the default of int(int) is used.

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