CODEWITHSUNDEEP
c++visual-c++referenceauto
jak jaki
jak jaki

Reputation: 41

auto reference in c++

This code give me strange debug info in visual studio 2015

int main() {
    const int i = 42;
    auto j = i; const auto &k = i; auto *p = &i;
    const auto j2 = i, &k2 = i;
}

The resulting types were:

&k  = const int &
&k2 = const int *

I think those should both be const int &.

Question is, why is my Visual Studio Debugger saying &k and &k2 are of different type?

Upvotes: 4

Views: 3263

Answers (1)

Bathsheba
Bathsheba

Reputation: 234715

k and k2 are both const int& types.

Here is the full type list. Note that top-level const is discarded for auto type deduction.

int main()
{
    const int i = 42;
    auto j = i; // i is an int (const is top-level)
    const auto &k = i; // k is a const int&
    auto *p = &i; // p is a const int* (const persists as not top-level).
    const auto j2 = i, &k2 = i; // j2 is a const int, k2 is a const int&
}

Finally, if you had written

auto q = &k2;

then the type of q is a const int*, since the const is not top-level so is not discarded but auto type deduction. This recovers the debug info you observe.

j2 and k2 look dissimilar but really that's due to how declarations work with the comma, cf. The confusion can be unpicked by writing

const int j2 = i, &k2 = i;

You can always find out for sure using the C++11 standard library function is_same: e.g.

bool am_I_the_same = std::is_same<decltype(k2), const int&)::value

Where decltype recovers the type of its argument.

Reference: http://en.cppreference.com/w/cpp/types/is_same

Upvotes: 1

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