C++11 lambda in decltype

c++ lambda c++11 decltype
HighCommander4 picture HighCommander4 · Jan 31, 2011 · Viewed 13.2k times · Source

For the following code:

auto F(int count) -> decltype([](int m) { return 0; }) 
{                                                               
    return [](int m) { return 0; };                                  
}

g++ 4.5 gives the errors:

test1.cpp:1:32: error: expected primary-expression before 'int'
test1.cpp:1:32: error: expected ')' before 'int'

What is the problem? What is the correct way to return a lambda from a function?

Answer

Puppy picture Puppy · Jan 31, 2011

You cannot use a lambda expression except by actually creating that object- that makes it impossible to pass to type deduction like decltype.

Ironically, of course, the lambda return rules make it so that you CAN return lambdas from lambdas, as there are some situations in which the return type doesn't have to be specified.

You only have two choices- return a polymorphic container such as std::function, or make F itself an actual lambda.

auto F = [](int count) { return [](int m) { return 0; }; };

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