When you add the const
keyword to a method the this
pointer will essentially become a pointer to const
object, and you cannot therefore change any member data. (Unless you use mutable
, more on that later).
The const
keyword is part of the functions signature which means that you can implement two similar methods, one which is called when the object is const
, and one that isn't.
#include
class MyClass
{
private:
int counter;
public:
void Foo()
{
std::cout << "Foo" << std::endl;
}
void Foo() const
{
std::cout << "Foo const" << std::endl;
}
};
int main()
{
MyClass cc;
const MyClass& ccc = cc;
cc.Foo();
ccc.Foo();
}
This will output
Foo
Foo const
In the non-const method you can change the instance members, which you cannot do in the const
version. If you change the method declaration in the above example to the code below you will get some errors.
void Foo()
{
counter++; //this works
std::cout << "Foo" << std::endl;
}
void Foo() const
{
counter++; //this will not compile
std::cout << "Foo const" << std::endl;
}
This is not completely true, because you can mark a member as mutable
and a const
method can then change it. It's mostly used for internal counters and stuff. The solution for that would be the below code.
#include
class MyClass
{
private:
mutable int counter;
public:
MyClass() : counter(0) {}
void Foo()
{
counter++;
std::cout << "Foo" << std::endl;
}
void Foo() const
{
counter++;
std::cout << "Foo const" << std::endl;
}
int GetInvocations() const
{
return counter;
}
};
int main(void)
{
MyClass cc;
const MyClass& ccc = cc;
cc.Foo();
ccc.Foo();
std::cout << "Foo has been invoked " << ccc.GetInvocations() << " times" << endl;
}
which would output
Foo
Foo const
Foo has been invoked 2 times
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