New type of auto-generated constructor in C++20

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The code below doesn't compile on GCC 11 with -std=c++17, but does with -std=c++20:

#include <iostream>
#include <string>

struct Foo {
    std::string s;
    int i;
};

int main()
{
    Foo foo("hello", 42);
    std::cout << foo.s << ' ' << foo.i << '\n';
}

What's the feature in C++20 that enables this? What kind of constructor is generated by the compiler?

c++20 c++
1 Answers

The C++20 feature being used here is the initialization of aggregates from parenthesis (P0960R3):

This paper proposes allowing initializing aggregates from a parenthesized list of values; that is, Aggr(val1, val2) would mean the same thing as Aggr{val1, val2}, except that narrowing conversions are allowed.

Here is an example from the above link:

struct A {
  int a;
  int&& r;
};

int f();
int n = 10;

A a1{1, f()};               // OK, lifetime is extended
A a2(1, f());               // well-formed, but dangling reference
A a3{1.0, 1};               // error: narrowing conversion
A a4(1.0, 1);               // well-formed, but dangling reference
A a5(1.0, std::move(n));    // OK

So, for your code to work with C++17, just replace the parenthesis with braces.

#include <iostream>
#include <string>

struct Foo {
    std::string s;
    int i;
};

int main()
{
    Foo foo{"hello", 42};
    std::cout << foo.s << ' ' << foo.i << '\n';
}
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