Why can't I create a vector of lambdas (of the same type) in C++11?
I was trying to create a vector of lambda, but failed:
auto ignore = [&]() { return 10; }; //1
std::vector<decltype(ignore)> v; //2
v.push_back([&]() { return 100; }); //3
Up to line #2, it compiles fine. But the line#3 gives compilation error:
error: no matching function for call to 'std::vector<main()::<lambda()>>::push_back(main()::<lambda()>)'
I don't want a vector of function pointers or vector of function objects. However, vector of function objects which encapsulate real lambda expressions, would work for me. Is this possible?
Every lambda has a different type —even if they have the same signature. You must use a run-time encapsulating container such as std::function if you want to do something like that.
eg:
std::vector<std::function<int()>> functors;
functors.push_back([&] { return 100; });
functors.push_back([&] { return 10; });
All lambda expressions have a different type, even if they are identical character-by-character. You're pushing a lambda of a different type (because it's another expression) into the vector, and that obviously won't work.
One solution is to make a vector of std::function<int()> instead.
auto ignore = [&]() { return 10; };
std::vector<std::function<int()>> v;
v.push_back(ignore);
v.push_back([&]() { return 100; });
On another note, it's not a good idea to use [&] when you're not capturing anything.
While what others have said is relevant, it is still possible to declare and use a vector of lambda, although it's not very useful:
auto lambda = [] { return 10; };
std::vector<decltype(lambda)> vector;
vector.push_back(lambda);
So, you can store any number of lambdas in there, so long as it's a copy/move of lambda !
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