[dcl.constexpr] Modernize example of constexpr-usable functions #6484
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Eisenwave
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| constexpr int uninit() { | ||
| struct { int a; } s; | ||
| return s.a; // error: uninitialized read of \tcode{s.a} | ||
| constexpr int stinit(int n) { // \tcode{stinit} is constexpr-usable, but \tcode{stinit} |
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The paragraph the example is attached to defines constexpr-suitable, so I think that's what you meant.
| constexpr int stinit(int n) { // \tcode{stinit} is constexpr-usable, but \tcode{stinit} | |
| constexpr int stinit(int n) { // \tcode{stinit} is constexpr-suitable, but \tcode{stinit} |
| int r = 1; | ||
| while (--n > 0) r *= x; | ||
| return r; | ||
| constexpr int uninit() { // \tcode{uninit} is constexpr-usable, but a program which calls |
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| constexpr int uninit() { // \tcode{uninit} is constexpr-usable, but a program which calls | |
| constexpr int uninit() { // \tcode{uninit} is constexpr-suitable, but a program which calls |
| } | ||
| return n; | ||
| constexpr std::generator<int> iota(int n = 0) { | ||
| while (true) co_yield n++; // error: \tcode{co_yield} cannot be used in a constexpr function |
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I think this example can be improved.
[expr.const] bans co_await and co_yield,
but [dcl.constexpr]p3 (the paragraph the example is attached to)
bans a constexpr function that is a coroutine.
So I think it might better to use co_return instead,
which isn't banned by [expr.const].
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This is about constexpr, not about [expr.const].
The focus should be on stuff that is ok/ill-formed because of [dcl.constexpr].
Having one (punchy) example of something that is ok by [dcl.constexpr] but ill-formed if invoked in a constant-expression context in all cases is fine. An unconditional throw-expression is good enough for that.
| constexpr int uninit() { // \tcode{uninit} is constexpr-usable, but a program which calls | ||
| struct { int a; } s; // \tcode{uninit} in a constant expression is ill-formed | ||
| return s.a; |
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| constexpr int uninit() { // \tcode{uninit} is constexpr-usable, but a program which calls | |
| struct { int a; } s; // \tcode{uninit} in a constant expression is ill-formed | |
| return s.a; | |
| constexpr int uninit() { // OK, but never produces a core constant expression | |
| struct { int a; } s; | |
| return s.a; // erroneous behavior: uninitialized read of \tcode{s.a} |
Edit: In C++26, the UB is changed to EB.
| }; | ||
| struct Derived : virtual Base { | ||
| constexpr Derived() = default; // error: constructor cannot be constexpr in a | ||
| }; // class or struct with virtual base classes |
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"class or struct" -> "class"
("class" covers all of union, class, struct)
jensmaurer
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Fixes #5718.
As explained in the issue, the example has become largely noise because of how many restrictions have been lifted in
constexprfunctions. The intuition of most users is probably thatabs,long_max,prevand others should beconstexpr, unless proven otherwise. This shift in mentality calls for examples which demonstrate what isn't constexpr-usable, rather than what is.