I am confused with some of the examples that I have come across with C++11 lambdas. For eg:
#include <iostream>
#include <string>
using namespace std;
int main()
{
cout << []()->string{return "Hello World 1!";}() << endl;
[]{cout << "Hello World 2!" << endl;}();
string result = [](const string& str)->string {return "Hello World " + str;}("2!");
cout << "Result: " << result << endl;
result = [](const string& str){return "Hello World " + str;}("3!");
cout << "Result: " << result << endl;
string s;
[&s](){s = "Hello World 4!";}; // does not work
cout << s << endl;
[&s](){s = "Hello World 4!";}(); // works!
cout << s << endl;
return 0;
}
I am unable to figure out what the parentheses at the end are doing. Are they instantiating, as a constructor, a lambda? Given that the template for a lambda is:
[capture_block](parameters) mutable exception_specification -> return_type {body}
it baffles me that those parentheses are required for those instances to work. Can someone explain what they are why they are required?
Well, given that a lambda expression is basically an anonymous function, the parentheses at the end do nothing more than just call this function. So
result = [](const string& str){return "Hello World " + str;}("3!");
is just equivalent to
auto lambda = [](const string& str){return "Hello World " + str;};
string result = lambda("3!");
This holds in the same way for a lambda with no arguments, like in
cout << []()->string{return "Hello World 1!";}() << endl;
which would otherwise (if not called) try to output a lambda expression, which in itself doesn't work. By calling it it just puts out the resulting std::string
.