MyClass a1 {a}; // clearer and less error-prone than the other three
MyClass a2 = {a};
MyClass a3 = a;
MyClass a4(a);
Why?
This question is related to
c++
c++11
syntax
initialization
list-initialization
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.
There are already great answers about the advantages of using list initialization, however my personal rule of thumb is NOT to use curly braces whenever possible, but instead make it dependent on the conceptual meaning:
In my experience, this ruleset can be applied much more consistently than using curly braces by default, but having to explicitly remember all the exceptions when they can't be used or have a different meaning than the "normal" function-call syntax with parenthesis (calls a different overload).
It e.g. fits nicely with standard library-types like std::vector:
vector<int> a{10,20}; //Curly braces -> fills the vector with the arguments
vector<int> b(10,20); //Parentheses -> uses arguments to parametrize some functionality,
vector<int> c(it1,it2); //like filling the vector with 10 integers or copying a range.
vector<int> d{}; //empty braces -> default constructs vector, which is equivalent
//to a vector that is filled with zero elements
There are MANY reasons to use brace initialization, but you should be aware that the initializer_list<> constructor is preferred to the other constructors, the exception being the default-constructor. This leads to problems with constructors and templates where the type T constructor can be either an initializer list or a plain old ctor.
struct Foo {
Foo() {}
Foo(std::initializer_list<Foo>) {
std::cout << "initializer list" << std::endl;
}
Foo(const Foo&) {
std::cout << "copy ctor" << std::endl;
}
};
int main() {
Foo a;
Foo b(a); // copy ctor
Foo c{a}; // copy ctor (init. list element) + initializer list!!!
}
Assuming you don't encounter such classes there is little reason not to use the intializer list.
It only safer as long as you don't build with -Wno-narrowing like say Google does in Chromium. If you do, then it is LESS safe. Without that flag the only unsafe cases will be fixed by C++20 though.
Note: A) Curly brackets are safer because they don't allow narrowing. B) Curly brackers are less safe because they can bypass private or deleted constructors, and call explicit marked constructors implicitly.
Those two combined means they are safer if what is inside is primitive constants, but less safe if they are objects (though fixed in C++20)
Source: Stackoverflow.com