In C++ what causes an assignment to evaluate as true or false when used in a control structure?

Question

So can someone help me grasp all the (or most of the relevant) situations of an assignment inside something like an if(...) or while(...), etc?

What I mean is like:<

Answer 1

if (a = b)
    ...

is the shorthand for:

a = b;
if (a != 0)
    ...

In a while statement, assignment within the condition enables the DRY principle:

while (a = b)
{
    ...
}

is shorthand for (notice how I had to replicate the assignment so that it is done right before the condition check):

a = b;
while (a != 0)
{
    ...
    a = b;
}

That said, one classic issue with code like this is knowing whether the code intended to do an assignment or if the code forget an '=' when the intent was to write '==' (i.e. should that have been while (a == b).

Because of this, you should never write just a plain assignment (gcc will issue a warning such as "suggest parentheses around assignment used as truth value"). If you want to use assignment in a control structure, you should always surround it with extra parentheses and explicitly add the not-equal:

if ((a = b) != 0)
    ...

while ((a = b) != 0)
    ...