Member Avatar for masa

Hi
I would like to know which is better. inialise a value like

int i=0;

or like this.

int i(0);

and why.

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Short answer for : for built-in scalar types there is no semantic difference between int i = 0; and int i(0);. For class types the two forms can behave differently because T x = y; is copy-initialization while T x(y); is direct-initialization; copy-initialization does not consider explicit constructors. was correct to call out explicit — that is the primary visible difference you’ll hit for user types. C++ initialization rules. (cppreference.net)

A clarification to : initialization constructs objects (constructors and, in copy-initialization, copy/move constructors); it does not call the assignment operator for a brand-new object. The assignment operator is used only when assigning into an already-initialized object. See how copy-initialization and constructors are selected. (cppreference.com)

Watch out for the “most vexing parse”: some parenthesized declarations are parsed as function declarations, not object creation. For example:

Widget w();   // declares a function, not an object
Widget w{};   // constructs an object (preferred in modern C++)

Using braces {} (uniform initialization) avoids that ambiguity. (w.cppreference.com)

Modern practical advice: prefer brace (list) initialization in C++11 and later for a consistent rule set and to prevent narrowing conversions, but be aware that brace-initialization may prefer std::initializer_list-taking constructors in overload resolution (so overload behavior can change). For details on list-initialization and narrowing rules, see list-initialization. ’s GOTW pointer is a good historical supplement to this modern view. (docs.cppreference.com)

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Member Avatar for Dave Sinkula

For native types I don't think it makes a difference. For user-defined types I think it may mean the difference (if any) between calling (the default constructor and [probably not in this case?]) the assignment operator and a copy constructor.

Real C++ people...?

Member Avatar for Narue

>I would like to know which is better.
There's no difference if they both compile (they might not for user-defined types), pick which you find more intuitive.

>Real C++ people...?
I don't think I count as a real C++ person, but both call the primary (non-copy) constructor. The only noticeable difference would be if the constructor in question is defined as explicit, in which case an implicit argument would be illegal:

class test1 {
public:
  test1 ( int init ) {}
};

class test2 {
public:
  explicit test2 ( int init ) {}
};

int main()
{
  test1 a1 ( 0 );
  test1 b1 = 0;
  test2 a2 ( 0 );
  test2 b2 = 0; // This will cause an error
}
Member Avatar for masa

thank you everyone for the answer

Member Avatar for Dave Sinkula

Thanks, I needed that.

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