C++: using curly braces to prevent narrowing during assignment

Question

I'm familiar with using curly braces/ initializer lists to prevent narrowing when initializing a variable, but is it good practice to use it when assigning a value to a variable too?

For e.g.

int i{1};       // initialize i to 1
double d{2.0};  // initialize d to 2.0
i = {2};        // assign value 2 to i
i = {d};        // error: narrowing from double to int

Is there a reason not to use curly braces for assignment?

Answer 1

Isn't a problem of initialization vs assignment.

It's a problem of different types.

If you try to initialize an int variable with a double, you get the same error.

And you can assign {d} to another double variable.

int main ()
 {
   int i{1};       // initialize i to 1
   //int i2{3.0};    // ERROR!
   double d{2.0};  // initialize d to 2.0  
   double d2{1.0}; // initialize d2 to 1.0
   i = {2};        // assign value 2 to i
   //i = {d};        // error: narrowing from double to int
   d2 = {d};       // OK

   return 0;
 }

Your example, enriched.

A good practice when assigning a value? Can be if you want to be sure not to lose precision.

An example: you can write a template assign() function in this way

template <typename X, typename Y>
void assign (X & x, Y const & y)
 { x = {y}; }

So you are sure to avoid narrowing

 // i is of type int
 assign(i, 23);    // OK
 assign(i, 11.2);  // ERROR! 

If (when) narrowing isn't a problem, you can avoid the curly braces.