45 lines
1.6 KiB
Plaintext
45 lines
1.6 KiB
Plaintext
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== Why is this an issue?
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{cpp}20 introduces the `consteval` keyword, which enforces that a function is always evaluated at compile time.
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`constexpr` functions can produce a compile-time constant in some contexts and when called with appropriate arguments, but they can also be invoked at run-time.
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`consteval` functions must always be evaluated at compile-time. If that cannot happen, a compilation error will occur.
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User-defined literal operators are designed to be called with constant operands known at compile time.
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Therefore, if these are intended to be evaluated at compile time with `constexpr`, `consteval` should be used instead to enforce compile time evaluation.
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This guarantees that no code is evaluated at run-time, and also enables error detection at compile-time.
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=== Noncompliant code example
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[source,cpp,diff-id=1,diff-type=noncompliant]
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----
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constexpr unsigned char operator ""_u8(unsigned long long value) {
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if (value >= 256u) { throw std::overflow_error("Overflow on literal"); }
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return static_cast<unsigned char>(value);
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}
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void f() {
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auto const a = 128_u8; // evaluated at compile time
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auto const b = 512_u8; // evaluated at runtime, throws when evaluated
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}
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----
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=== Compliant solution
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[source,cpp,diff-id=1,diff-type=compliant]
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----
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consteval unsigned char operator ""_u8(unsigned long long value) {
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if (value >= 256u) { throw std::overflow_error("Overflow on literal"); }
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return static_cast<unsigned char>(value);
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}
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void f() {
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auto const a = 128_u8; // evaluated at compile time
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auto const b = 512_u8; // compilation error, the bug is detected
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}
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----
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