c++ - Why does std::span::operator[] not implement a const reference? - Stack Overflow

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When writing:

std::vector<double> vec{1.0,2.0};
std::span<const double> s(vec);
s[0]=1.0;

I get a compiler error, as expected:

Cannot assign to return value because function 'operator[]' returns a const value

However, when I go and see the return type of std::span::operator[], I only see a reference return, not a const reference return, while I was expecting a decltype(auto) for the return type.

Why do I not get an error that the operator with const reference is not implemented? How does the compiler figure out the correct answer?

When writing:

std::vector<double> vec{1.0,2.0};
std::span<const double> s(vec);
s[0]=1.0;

I get a compiler error, as expected:

Cannot assign to return value because function 'operator[]' returns a const value

However, when I go and see the return type of std::span::operator[], I only see a reference return, not a const reference return, while I was expecting a decltype(auto) for the return type.

Why do I not get an error that the operator with const reference is not implemented? How does the compiler figure out the correct answer?

• c++
• reference
• const-reference
• std-span

Share Improve this question Follow edited Mar 28 at 9:22 Jarod42 219k1515 gold badges196196 silver badges330330 bronze badges asked Mar 27 at 20:29 RomainRomain 4111 silver badge44 bronze badges 0

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1 Answer 1

Sorted by: Reset to default 6

If the type of T is const double, then the return of T& is const double&.

The qualifiers, like const or volatile are part of the type that is held by the span, and are transitive to the returned reference of the operator[].

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