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c++language-lawyerc++20stdvectorconsteval

Returning std::vector from an immediate function

Since C++20, std::vector can be used in constant expressions. And as far as I known, current C++ permits dynamically allocate memory under the condition that any such allocation is deallocated by the time the constant expression is "over".

However, I encounter that in case of immediate function the rules might be different. Please consider the example:

consteval auto getVec() {
    return std::vector<int>(9);
}

static_assert( getVec().size() == 9 );

Here immediate consteval function getVec returns not-empty std::vector, size of which is verified in a constant expression.

I expected that this code would compile, because all deallocations must be done automatically, and indeed it is accepted in Clang with libc++.

But MSVC complains:

error C7595: 'getVec': call to immediate function is not a constant expression
note: (sub-)object points to memory which was heap allocated during constant evaluation
fatal error C1903: unable to recover from previous error(s); stopping compilation

and GCC behaves similarly:

error: 'getVec()()' is not a constant expression because it refers to a result of 'operator new'

Online demo: https://gcc.godbolt.org/z/d736qr3hh

Which compiler is correct here?

Solution

  • This is fixed with P2564R3 being accepted as a defect report for C++20. It allows getVec() to be a subexpression of a larger constant expression without being a constant expression by itself.

    LLVM 17 and the upcomming GCC 14 implement it. For MSVC there is a feature request.

    The original reason for the problem was this:

    [expr.prim.id]p3:

    A potentially-evaluated id-expression that denotes an immediate function ([dcl.constexpr]) shall appear only

    • as a subexpression of an immediate invocation, or
    • in an immediate function context ([expr.const])

    [expr.const]p13:

    An expression or conversion is in an immediate function context if it is potentially evaluated and its innermost non-block scope is a function parameter scope of an immediate function. An expression or conversion is an immediate invocation if it is potentially-evaluated explicit or implicit invocation of an immediate function and is not in an immediate function context. An immediate invocation shall be a constant expression.

    static_assert( getVec().size() == 9 ); is not in an immediate function context. Therefore, the explicit invocation getVec() is an immediate invocation, so it needs to produce a constant expression. getVec() by itself is not a constant expression since it doesn't deallocate the memory allocated by new.

    For comparison, the following do compile https://godbolt.org/z/9KqhY7oP8

    consteval auto getVec() {
    return std::vector<int>(9);
}

consteval auto getVecSize() {
    return getVec().size();
}

static_assert(getVecSize() == 9);
static_assert([]() consteval { return getVec().size() == 9; }());

consteval void InAnImmediateFunctionContext() {
    static_assert(getVec().size() == 9);
}