Can You Use a Lambda In A Class' Initialization List?

Question

I am trying to use a C++11 Lambda to initialize a const member variable of a class.

A much simplified example:

class Foo { public: const int n_; Foo(); }; Foo::Foo() : n_( []() -> int { return 42; } ) { } int main() { Foo f; }

In MSVC10 this yields:

error C2440: 'initializing' : cannot convert from '`anonymous-namespace'::<lambda0>' to 'const int'

In IDEONE this yields:

prog.cpp: In constructor 'Foo::Foo()': prog.cpp:9:34: error: invalid conversion from 'int (*)()' to 'int'

I'm starting to get the idea that I can't use lambdas in a class' initialization list.

Can I? If so, what's the proper syntax?

stijn · Accepted Answer · 2012-06-14 16:45:51Z

you are trying to convert from a lambda to int - you should call the lambda instead:

Foo::Foo() : n_( []() -> int { return 42; }() ) //note the () to call the lambda! { }

Konrad Rudolph · Accepted Answer · 2012-06-14 16:45:15Z

7

Your variable is declared as int.

Do you want to invoke the lambda? This should work:

n_(([]() -> int { return 42; })())

Or did you want a variable of type std::function<>?

answered Jun 14, 2012 at 16:45

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3 Comments

+1: Yes, I am trying to populate n_ by returning the value from the lambda.

@John See update. And I actually see how that can be useful with a more complex (multi-statement) lambda.

Exactly. In my actual use case, I am storing the size of a read-only file in bytes.

Mohammad · Accepted Answer · 2012-06-14 16:46:00Z

You're creating a lambda, this way as compiler states, you're trying to store the lambda itself in n_.