c++ - Is it safe to cast a lambda function to a function pointer?

Question

I have this code:

void foo(void (*bar)()) {
    bar();
}

int main() {
    foo([] {
        int x = 2;
    });
}

However, I'm worried that this will suffer the same fate as:

struct X { int i; };

void foo(X* x) {
    x->i = 2;
}

int main() {
    foo(&X());
}

Which takes the address of a local variable.

Is the first example completely safe?

Answer 1

A lambda that captures nothing is implicitly convertible to a function pointer with its same argument list and return type. Only capture-less lambdas can do this; if it captures anything, then they can't.

Unless you're using VS2010, which didn't implement that part of the standard, since it didn't exist yet when they were writing their compiler.