It's a variable declaration with an initialiser. 

$ cat a.cpp
#include <stdio.h>

struct Point {
   Point(int a_x, int a_y) : x(a_x), y(a_y) {}
   int x;
   int y;
};

int main() {
   Point point(5,6);
   printf("%d,%d\n", point.x, point.y);
   return 0;
}

$ g++ -o a a.cpp && a
5,6

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It's like new, but the object is allocated on the stack instead of the heap, and the variable is a reference instead of a pointer. It's the object equivalent of 

int i = 5;

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which can also be written as 

int i(5);

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which is what allowed me to write 

Point(int a_x, int a_y) : x(a_x), y(a_y) {}

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instead of 

Point(int a_x, int a_y) { x = a_x; y = a_y; }

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In reply to Re^4: returning an object in XS by ikegami
in thread returning an object in XS by bipham

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