How can I initialize a std::map with comparison lambda by using an initializer list?

Question

I can use an initializer list to initialize a std::map as follows:

std::map<int, int> m {{5, 6}, {3, 4}, {1, 2}};

I can change the ordering of a std::map by providing a comparison lambda (see here, search for "lambda") as follows:

auto comp = [](int a, int b) { return b < a; };
std::map<int, int, decltype(comp)> m(comp);

Now, I tried to do both at the same time as follows:

std::map<int, int, decltype(comp)> m(comp) {{5, 6}, {3, 4}, {1, 2}};

However, this does not compile. On VS 2013 I'm getting the following error:

error C2448: 'm' : function-style initializer appears to be a function definition

I also tried running the code on Ideone, but there I'm getting the following error:

error: expected ‘}’ at end of input

This looks like some kind of most vexing parse to me. I tried to provide an assignment operator or use std::make_pair within the initializer list, but to no avail.

How can I use an initializer list here? Is it possible at all?

Answer 1

The constructor which takes an initializer list and a comparator is the following:

map( std::initializer_list<value_type> init,
     const Compare& comp = Compare(),
     const Allocator& alloc = Allocator() );

So you should write:

auto comp = [](int a, int b) { return b < a; };
std::map<int, int, decltype(comp)> m{{{5, 6}, {3, 4}, {1, 2}}, comp};