only input data is modified module main; import std.stdio; import std.process; void Function(in uint[int]o_a){ foreach(l_a; o_a.keys)o_a.remove(l_a); } void main(char [][] args){ uint[int]l_a; l_a[1] = 0; l_a[2] = 3; writefln(l_a.length); // 2 Function(l_a); writefln(l_a.length); // 0 system("pause"); }