Properly initialising variables in modern C++ (C++11 and above), using () or {}?

Question

The C++ reference pages say that () is for value initialisation, {} is for value and aggregate and list initialisation. So, if I just want value initialisation, which one do

Answer 1

There is another important difference: The brace initializer requires that the given type can actually hold the given value. In other words, it forbids narrowing of the value, like rounding or truncation.

int a(2.3); // ok? a will hold the value 2, no error, maybe compiler warning
uint8_t c(256); // ok? the compiler should warn about something fishy going on

As compared to the brace initialization

int A{2.3}; // compiler error, because int can NOT hold a floating point value
double B{2.3}; // ok, double can hold this value
uint8_t C{256}; // compiler error, because 8bit is not wide enough for this number

Especially in generic programming with templates you should therefore use brace initialization to avoid nasty surprises when the underlying type does something unexpected to your input values.