#defining constants in C++

Question

In various C code, I see constants defined like this:

#define T 100

Whereas in C++ examples, it is almost always:

const int T = 100;

It is my understanding that in the first case, the preprocessor will replace every instance of T with 100. In the second example, T is actually stored in memory.

Is it considered bad programming practice to #define constants in C++?

Answer 1

Is it considered bad programming practice to #define constants in C++?

Yes, because all macros (which are what #defines define) are in a single namespace and they take effect everywhere. Variables, including const-qualified variables, can be encapsulated in classes and namespaces.

Macros are used in C because in C, a const-qualified variable is not actually a constant, it is just a variable that cannot be modified. A const-qualified variable cannot appear in a constant expression, so it can't be used as an array size, for example.

In C++, a const-qualified object that is initialized with a constant expression (like const int x = 5 * 2;) is a constant and can be used in a constant expression, so you can and should use them.