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

28

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!
{
}

answered Jun 14, 2012 at 16:45

stijn

36.1k13 gold badges119 silver badges167 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

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

Konrad Rudolph

549k142 gold badges967 silver badges1.3k bronze badges

3 Comments

John Dibling Over a year ago

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

Konrad Rudolph Over a year ago

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

John Dibling Over a year ago

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

2

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

answered Jun 14, 2012 at 16:46

Mohammad

1,2521 gold badge11 silver badges26 bronze badges

Collectives™ on Stack Overflow

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

3 Answers 3

Comments

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related