Initializing multiple variables in C++

 Initializing multiple variables

In the last section, we noted that it is possible to define multiple variables of the same type in a single statement by separating the names with a comma:

int a, b;

We also noted that best practice is to avoid this syntax altogether. However, since you may encounter other code that uses this style, it’s still useful to talk a little bit more about it, if for no other reason than to reinforce some of the reasons you should be avoiding it.

You can initialize multiple variables defined on the same line:

int a = 5, b = 6; // copy initialization
int c( 7 ), d( 8 ); // direct initialization
int e { 9 }, f { 10 }; // brace initialization (preferred)

Unfortunately, there’s a common pitfall here that can occur when the programmer mistakenly tries to initialize both variables by using one initialization statement:

int a, b = 5; // wrong (a is not initialized!)

int a = 5, b = 5; // correct

In the top statement, variable “a” will be left uninitialized, and the compiler may or may not complain. If it doesn’t, this is a great way to have your program intermittently crash or produce sporadic results. We’ll talk more about what happens if you use uninitialized variables shortly.

The best way to remember that this is wrong is to consider the case of direct initialization or brace initialization:

int a, b( 5 );
int c, d{ 5 };

Because the parenthesis or braces are typically placed right next to the variable name, this makes it seem a little more clear that the value 5 is only being used to initialize variable b and d, not a or c.