CODEWITHSUNDEEP
c++c++11
mike
mike

Reputation: 1720

What is this use of auto ? - ADL?

I have always thought that auto should be used in the following form auto varName = someOtherVar;. Today I have found that I could also use auto varName(someOtherVar);. At first I thought that maybe this is Argument Depending Lookup at work. But I am not sure. What are the restrictions for using such auto syntax? Below is some code:

#include <iostream>
#include <string>
#include <vector>

class Person {
    public:
    Person(std::string s) : name(s) {}
    Person(const Person& p) : name(p.name) {}
    std::string name;
};

int main()
{
    Person p("hello");
    auto p2(p); // equivalent to auto p2 = p; ?
    std::cout << p2.name;
}

Upvotes: 0

Views: 63

Answers (1)

M.M
M.M

Reputation: 141554

auto may be used in place of the type specifier in a declaration with initializer; then the type is deduced from the initializer.

T a = b; is called copy initialization , T a(b); is called direct initialization , there are subtle differences.

So auto p2(p); is the same as Person p2(p);. Since the initializer has the same type as the object being initialized, copy-initialization and direct-initialization are identical in this case.

"Argument-dependent lookup" refers to resolving the scope of a function name. But this is an object declaration, not a function call. (There is ADL on the line std::cout << p2.name; though - operator<< resolves via ADL to std::operator<<).

Upvotes: 2

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