Skip to main content

#3. The condition in an "if" statement should be a simple boolean expression with no side effects

The expression inside an if statement should be boolean, simple, and have no side effects.

In C++, every statement is an expression of a certain type. It may be an ordinary type, such as boolint or a user-defined type such as Customer), or a special type called void which basically means "no type".

In C++ the expression inside the pair of brackets will be understood as a boolean expression. Some people carry the habit from their C days and pass an as int an argument to the if. Other people even use a pointer, so that the if will check whether it is NULL and will evaluate to true in that case. Don't do it - it makes the code less readable and a step closer to being error prone.

Guideline #2 advised you to avoid implicit type conversions. This is a very good place to start applying it. The if statement requires a bool, so why pass it anything else?

Don't overcomplicate things by putting several actions together. Don't use complex commands which may succeed or not, returning a boolean which indicates success, as conditions inside if statements. Don't write assignments or, worse, variable declarations in the condition. In short, keep it simple.

It's better to separate the list of things that you need to do (check whether the object, whatever it is is connected, and then save to file) from a conditional action which you will perform if the result of one of these actions is not satisfactory.


[McConnell 2004]
Steve McConnell: Code Complete, 2nd Edition, Microsoft Press, 2004.

This book discusses conditionals in Chapter 15. In particular, see Section 15.1 "if statements" (pages 355-360) and the checklist at the end of the chapter (page 365).


Popular posts from this blog

#11. Don't use syntactic overloading

Syntactic overloading (or function overloading) is a C++ feature which allows a class to have several functions with the same name, but with different parameters. To say "Don't use syntactic overloading" is the same as to say "Don't give the same name to different functions".

class C
    void add(int i);
    void add(double d);
    void add(C other);

When you write this client code:

C c;

The compiler will call the appropiate version of the funcion add, depending on the type of x.

This is what syntactic overloading is. It is called syntactic overloading to distinguish it from semantic overloading 8more properly called dynamic binding), which is the one that happens when you use inheritance and polymorphism.

Syntactic overloading may seem useful at first sight, but it has lots of drawbacks for no real advantage. It just can go wrong in too many ways.

Code is more readable without syntactic overloading. One name for each thing, different n…

#2. Make all type conversions explicit

Assignments, function calls and expressions in general should not contain implicit type conversions. All type conversions should be explicit.

Type conversions should be isolated in their own line, which should consist only of the assignment of the result of the type conversion to a variable of the target type.

A statement should not mix type conversion with other operations. For example, if you need to convert an object to one of another type to pass it as a function parameter, do not do it in the function call itself; instead, do it in a separate instruction in the previous line.
Type conversion occurs when an entity of type A is, well, converted to an entity of type B. This may occur in function calls, expressions using operators, or assignments.
An explicit type conversion is one which you can read in the code. An implicit type conversion is one which does not appear in the source code, but is done automatically when the software is executed.
Making all type conversions explicit he…

#10. Choosing the right loop structure

C++ has three loop instructions: for, while and do ... while. To choose wisely among them, you need to know what their differences are.
1) for A for loop makes sense when you repeat an action for a known number of times, or for all the elements of a known set.

This is the syntax of a forloop:

for (initialization; condition; expression)

statement is actually a compound statement, that is, a block of code. This is called the body of the loop. In contrast, the initialization, condition and expression together are called the header of the loop. The initialization is a single statement which ends at the semicolon I wrote after it. The condition is a boolean expression. The expression is a statement. It should be a single statement which modifies one variable which is involved in the condition (see #guideline #5).

The initialization is performed. Then, the condition is evaluated. Then, two things can happen. If the condition is false, the code jumps right after the for statemen…