'Excess elements in struct initializer' error with C++11 uniform initialization

Question

I am surprised by the following compiler error:

template <typename T>
struct A
{
    A(T t): t_{t} {}

    T t_;
};

struct S
{
};

int main()
{
    A<S> s{S{}};
}

The error is (with clang):

test.cpp:4:16: error: excess elements in struct initializer
    A(T t): t_{t} {}
               ^
test.cpp:15:10: note: in instantiation of member function 'A<S>::A' requested here
    A<S> s{S{}};
         ^

GCC gives a similar error.

I would expect the expression t_{t} to try to copy construct t_ from t. Since S has an implicitly generated copy constructor, I wouldn't expect this to be a problem.

Could someone explain what is going on here?

Nicol Bolas · Accepted Answer · 2017-03-20 17:27:56Z

18

S may have an implicitly generated copy constructor, but S is also something else. An aggregate. Therefore, (almost) any use of {} will perform aggregate initialization on it. So the contents of {} are expected to be values for the members of the aggregate. And since your aggregate is empty... boom.

In template code, uniform initialization syntax should be avoided for exactly these reasons. For an unknown type T, you can't be sure exactly what {...} will do.

edited Mar 20, 2017 at 17:27

answered Feb 20, 2013 at 22:13

Nicol Bolas

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5 Comments

ipc Over a year ago

uniform initialization syntax should be avoided for exactly these reasons ... and for many other reasons like changing semantics when modifying S.

David Rodríguez - dribeas Over a year ago

@ipc: I would not go as far, but the fact is that it can be tricky at times :)

Nicol Bolas Over a year ago

@zahir: Yes. Some of us would like to fix that, but there seems to be strong opposition to the very idea of uniform initialization, let alone to actually correcting the problem and making it uniform.

Johannes Schaub - litb Over a year ago

There exists a core issue for this, too.

gnzlbg Over a year ago

@JohannesSchaub-litb do you know if it was resolved in C++14?

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