Monday, January 4, 2010

"this" as implicit arg to non-static c++ methods

Compiler adds an implicit param to each non-static method. Note in both java and c++, non-static method invocation absolutely[2] needs a receiving object ie the "this" pointer. C++ merely add that pointer as the implicit argument. In the example below, the const object is passed as argument to the method as if (suppose j1 is an instance of class J)

m(this, other_args) -- binds to --> m(J*, other_params)

Method call performs an pbref

J* this_ptr = & j1;

Since constness radiates left[1], a const j1 (RHS) requires the param (LHS) declared const. You can mark m() const, since it's interpreted as

m(const J*, other_params), so now the method call does a pbref

const J* this_ptr = & j1;

[1] see other posts
[2] There's simply no exception. A non-static method invocation is a message, and needs an recipient object.

using namespace std;
template<class T> struct J{
J(T & rhs){}
void m(T a) {
this->intVar = a; // legal when method is const but no one calls.
}
T intVar;
};
int main()
{
int a=22;
const J<int> j1(a);
j1.m(89);
return 0;
}

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