When a float variable goes out of the float limits, what happens?

Formally, the behavior is undefined. On a machine with IEEEfloating point, however, overflow after rounding will resultin Inf. The precision is limited, however, and the resultsafter rounding of FLT_MAX + 1 are FLT_MAX.

You can see the same effect with values well under FLT_MAX.Try something like:

float f1 = 1e20;     // less than FLT_MAXfloat f2 = f1 + 1.0;if ( f1 == f2 ) ...

The if will evaluate to true, at least with IEEE arithmetic.(There do exist, or at least have existed, machines wherefloat has enough precision for the if to evaluate tofalse, but they aren't very common today.)

It depends on what you are doing. If the float "overflow" comes in an expression which is directly returned, i.e.

return std::numeric_limits::max() + std::numeric_limits::max();

the operation might not result in an overflow. I cite from the C standard [ISO/IEC 9899:2011]:

The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not apply to the case of function return. The representation of floating-point values may have wider range or precision than implied by the type; a cast may be used to remove this extra range and precision.

See here for more details.