MyClass a1 {a}; // clearer and less error-prone than the other three
MyClass a2 = {a};
MyClass a3 = a;
MyClass a4(a);
Why?
Basically copying and pasting from Bjarne Stroustrup's "The C++ Programming Language 4th Edition":
List initialization does not allow narrowing (§iso.8.5.4). That is:
Example:
void fun(double val, int val2) {
int x2 = val; // if val==7.9, x2 becomes 7 (bad)
char c2 = val2; // if val2==1025, c2 becomes 1 (bad)
int x3 {val}; // error: possible truncation (good)
char c3 {val2}; // error: possible narrowing (good)
char c4 {24}; // OK: 24 can be represented exactly as a char (good)
char c5 {264}; // error (assuming 8-bit chars): 264 cannot be
// represented as a char (good)
int x4 {2.0}; // error: no double to int value conversion (good)
}
The only situation where = is preferred over {} is when using auto
keyword to get the type determined by the initializer.
Example:
auto z1 {99}; // z1 is an int
auto z2 = {99}; // z2 is std::initializer_list<int>
auto z3 = 99; // z3 is an int
Prefer {} initialization over alternatives unless you have a strong reason not to.