Is !! a safe way to convert to bool in C++?

Question

[This question is related to but not the same as this one.]

If I try to use values of certain types as boolean expressions, I get a warning. Rather than su

Answer 1

I would not use:

bool b = !!t;

That is the least readable way (and thus the hardest to maintain)

The others depend on the situation.
If you are converting to use in a bool expression only.

bool b = t ? true : false;
if (b)
{
    doSomething();
}

Then I would let the language do it for you:

if (t)
{
    doSomething();
}

If you are actually storing a boolean value. Then first I would wonder why you have a long in the first places that requires the cast. Assuming you need the long and the bool value I would consider all the following depending on the situation.

bool  b = t ? true : false;      // Short and too the point.
                                 // But not everybody groks this especially beginners.
bool  b = (t != 0);              // Gives the exact meaning of what you want to do.
bool  b = static_cast(t);  // Implies that t has no semantic meaning
                                 // except as a bool in this context.

Summary: Use what provides the most meaning for the context you are in.
Try and make it obvious what you are doing