Declaring and initializing a variable in a Conditional or Control statement in C++

Question

In Stroustrup\'s The C++ Programming Language: Special Edition (3rd Ed), Stroustrup writes that the declaration and initialization of variables in the conditionals

Answer 1

I consider it a good style when used with possibly NULL pointer:

if(CObj* p = GetOptionalValue()) {
   //Do something with p
}

This way whether p is declared, it is a valid pointer. No dangling pointer access danger.

On the other hand at least in VC++ it is the only use supported (i.e. checking whether assignment is true)

Answer 2

They are fixing this in c++17 :

if (int i = read(socket); i < 0)

where if can have an initializer statement.

see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r0.html

Answer 3

I've run into a similar problem:

The problem seems to be the parentheses around the int declaration. It should work if you can express the assignment and test without them, i.e.

if (int i = read(socket)) {

should work, but that means the test is != 0, which is not what you want.