Specifying a lambda function as default argument

Question

How do I assign a lambda as default argument? I would like to do this:

int foo(int i, std::function<int(int)> f = [](int x) -> int { return x / 2; }) {     return f(i); } 

but my compiler (g++ 4.6 on Mac OS X) complains:

error: local variable 'x' may not appear in this context 

EDIT: Indeed, this was a compiler bug. The above code works fine with a recent version of gcc (4.7-20120225).

Answer 1

You could use overloading:

int foo(int i) {     return foo(i, [](int x) -> int { return x / 2; }); }  int foo(int i, std::function<int(int)> f) {     return f(i); } 

Answer 2

This seems to be a bug in gcc; the standard permits lambda expressions in default parameters as long as nothing is captured.

The following seems to be everything the FDIS says about lambdas in default parameters, so any use other than what is forbidden here ought to be permitted by default.

C++11 FDIS 5.1.2/13

A lambda-expression appearing in a default argument shall not implicitly or explicitly capture any entity.

[ Example:

void f2() {     int i = 1;     void g1(int = ([i]{ return i; })());       // ill-formed     void g2(int = ([i]{ return 0; })());       // ill-formed     void g3(int = ([=]{ return i; })());       // ill-formed     void g4(int = ([=]{ return 0; })());       // OK     void g5(int = ([]{ return sizeof i; })()); // OK } 

— end example ]