Expression must have class type
Summary: Instead of a.f();
it should be a->f();
In main you have defined a as a pointer to object of A, so you can access functions using the ->
operator.
An alternate, but less readable way is (*a).f()
a.f()
could have been used to access f(), if a was declared as:
A a;
a
is a pointer. You need to use->
, not .
It's a pointer, so instead try:
a->f();
Basically the operator .
(used to access an object's fields and methods) is used on objects and references, so:
A a;
a.f();
A& ref = a;
ref.f();
If you have a pointer type, you have to dereference it first to obtain a reference:
A* ptr = new A();
(*ptr).f();
ptr->f();
The a->b
notation is usually just a shorthand for (*a).b
.
A note on smart pointers
The operator->
can be overloaded, which is notably used by smart pointers. When you're using smart pointers, then you also use ->
to refer to the pointed object:
auto ptr = make_unique<A>();
ptr->f();
Allow an analysis.
#include <iostream> // not #include "iostream"
using namespace std; // in this case okay, but never do that in header files
class A
{
public:
void f() { cout<<"f()\n"; }
};
int main()
{
/*
// A a; //this works
A *a = new A(); //this doesn't
a.f(); // "f has not been declared"
*/ // below
// system("pause"); <-- Don't do this. It is non-portable code. I guess your
// teacher told you this?
// Better: In your IDE there is prolly an option somewhere
// to not close the terminal/console-window.
// If you compile on a CLI, it is not needed at all.
}
As a general advice:
0) Prefer automatic variables
int a;
MyClass myInstance;
std::vector<int> myIntVector;
1) If you need data sharing on big objects down
the call hierarchy, prefer references:
void foo (std::vector<int> const &input) {...}
void bar () {
std::vector<int> something;
...
foo (something);
}
2) If you need data sharing up the call hierarchy, prefer smart-pointers
that automatically manage deletion and reference counting.
3) If you need an array, use std::vector<> instead in most cases.
std::vector<> is ought to be the one default container.
4) I've yet to find a good reason for blank pointers.
-> Hard to get right exception safe
class Foo {
Foo () : a(new int[512]), b(new int[512]) {}
~Foo() {
delete [] b;
delete [] a;
}
};
-> if the second new[] fails, Foo leaks memory, because the
destructor is never called. Avoid this easily by using
one of the standard containers, like std::vector, or
smart-pointers.
As a rule of thumb: If you need to manage memory on your own, there is generally a superiour manager or alternative available already, one that follows the RAII principle.