is it possible to initialize static variable with lambda?

Question

I've tried this:

#include <map>

int main() {

  static std::map<int,int> myMap = [](){
    std::map<int,int> myMap;
    return myMap;
  };

}

error:

<stdin>: In function 'int main()':
<stdin>:8:3: error: conversion from 'main()::<lambda()>' to non-scalar type 'std::map<int, int>' requested

And yes, I know that I can create another 'normal' function for it ant it works but lambdas cannot initialize objects in that way.

ipc · Accepted Answer · 2013-03-22 23:57:11Z

45

Yes, it is indeed possible.

static std::map<int,int> myMap = [](){
  std::map<int,int> myMap;
  return myMap;
}();

Note the () at the end. You are assigning myMap to a lambda, but you really want to assign it to the result of the lambda. You have to call it for that.

answered Mar 22, 2013 at 23:57

ipc

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3 Comments

kostrykin Over a year ago

I think you missed the ->std::map<int,int> within the lambda declaration.

Adam H. Peterson Over a year ago

@theV0ID, the return type isn't necessary on a lambda if it can be deduced from the return statement (as it can here). (Side note: If the parameter list is empty, as it is here, it is also optional. Thus, this could be written using: static auto myMap = []{ return std::map<int,int>(); }();)

Th. Thielemann Over a year ago

Yes it is. Tested with gcc 4.8, running 5 threads and a static variable within the clousure. [](){ static int i = 0; i += 1; assert(i == 1); }

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