Reputation: 39859
MyClass a1 {a}; // clearer and less error-prone than the other three
MyClass a2 = {a};
MyClass a3 = a;
MyClass a4(a);
Why?
Upvotes: 646
Views: 336683
Reputation: 11418
Update (2022-02-11): Note that there are more recent opinions on that subject to the one originally posted (below), which argue against the preference of the {} initializer, such as Arthur O'Dwyer in his blog post on The Knightmare of Initialization in C++.
Original Answer:
Read Herb Sutter's (updated) GotW #1. This explains in detail the difference between these, and a few more options, along with several gotchas that are relevant for distinguishing the behavior of the different options.
The gist/copied from section 4:
When should you use ( ) vs. { } syntax to initialize objects? Why? Here’s the simple guideline:
Guideline: Prefer to use initialization with { }, such as vector v = { 1, 2, 3, 4 }; or auto v = vector{ 1, 2, 3, 4 };, because it’s more consistent, more correct, and avoids having to know about old-style pitfalls at all. In single-argument cases where you prefer to see only the = sign, such as int i = 42; and auto x = anything; omitting the braces is fine. …
That covers the vast majority of cases. There is only one main exception:
… In rare cases, such as vector v(10,20); or auto v = vector(10,20);, use initialization with ( ) to explicitly call a constructor that is otherwise hidden by an initializer_list constructor.
However, the reason this should be generally “rare” is because default and copy construction are already special and work fine with { }, and good class design now mostly avoids the resort-to-( ) case for user-defined constructors because of this final design guideline:
Guideline: When you design a class, avoid providing a constructor that ambiguously overloads with an initializer_list constructor, so that users won’t need to use ( ) to reach such a hidden constructor.
Also see the Core Guidelines on that subject: ES.23: Prefer the {}-initializer syntax.
Upvotes: 17
Reputation: 564
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:
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)
Upvotes: 7
Reputation: 21166
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
Upvotes: 203
Reputation: 39859
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.
Upvotes: 565
Reputation: 6047
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.
Upvotes: 125