Item 48:  Pay attention to compiler warnings.

Many programmers routinely ignore compiler warnings. After all, if the problem were serious, it'd be an error, right? This kind of thinking may be relatively harmless in other languages, but in C++, it's a good bet compiler writers have a better grasp of what's going on than you do. For example, here's an error everybody makes at one time or another:

class B {
public:
  virtual void f() const;
};

class D: public B {
public:
  virtual void f();
};

The idea is for D::f to redefine the virtual function B::f, but there's a mistake: in B, f is a const member function, but in D it's not declared const. One compiler I know says this about that:

warning: D::f() hides virtual B::f()

Too many inexperienced programmers respond to this message by saying to themselves, "Of course D::f hides B::f — that's what it's supposed to do!" Wrong. What this compiler is trying to tell you is that the f declared in B has not been redeclared in D, it's been hidden entirely (see Item 50 for a description of why this is so). Ignoring this compiler warning will almost certainly lead to erroneous program behavior, followed by a lot of debugging to find out about something that this compiler detected in the first place.

After you gain experience with the warning messages from a particular compiler, of course, you'll learn to understand what the different messages mean (which is often very different from what they seem to mean, alas). Once you have that experience, there may be a whole range of warnings you'll choose to ignore. That's fine, but it's important to make sure that before you dismiss a warning, you understand exactly what it's trying to tell you.

As long as we're on the topic of warnings, recall that warnings are inherently implementation-dependent, so it's not a good idea to get sloppy in your programming, relying on compilers to spot your mistakes for you. The function-hiding code above, for instance, goes through a different (but widely used) compiler with nary a squawk. Compilers are supposed to translate C++ into an executable format, not act as your personal safety net. You want that kind of safety? Program in Ada.