C++ – Why declare a variable in one line, and assign to it in the next

c

I often see in C and C++ code the following convention:

some_type val;
val = something;

some_type *ptr = NULL;
ptr = &something_else;

instead of 

some_type val = something;
some_type *ptr = &something_else;

I initially assumed that this was a habit left over from the days when you had to declare all local variables at the top of the scope. But I've learned not to dismiss so quickly the habits of veteran developers. So, is there a good reason for declaring in one line, and assigning afterwards?

Best Answer

C

In C89 all declarations had to be be at the beginning of a scope ({ ... }), but this requirement was dropped quickly (first with compiler extensions and later with the standard).

C++

These examples are not the same. some_type val = something; calls the copy constructor while val = something; calls the default constructor and then the operator= function. This difference is often critical.

Habits

Some people prefer to first declare variables and later define them, in the case they are reformatting their code later with the declarations in one spot and the definition in an other.

About the pointers, some people just have the habit to initialize every pointer to NULL or nullptr, no matter what they do with that pointer.

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