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Questions tagged [constant-expression]

Constant expressions can be evaluated at compile time.

Many languages require certain initializers to be constant expressions. For example:

  • Array bounds
  • Selectors in case statements
  • Bit-field length specification
  • Enumeration initializers
  • Non-type template arguments

A greatly restricted set of operands are allowed in constant expressions. Generally, variables are not allowed. For example, C++ allows:

  • Literals
  • Enumeration constants
  • Values declared as const that are initialized with constant expressions
  • sizeof expressions

Questions with this tag usually need help with error messages that indicate lines that require constant expressions.

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"Constant expressions" prior to C++11

The constexpr keyword was introduced in C++11, as (I think) was the corresponding idea of "constant expressions." However, this concept was implicitly present in C++98/c++03, since array declarations require a constant expression: // valid: int…
Kyle Strand
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Confusion about constant expressions

This is some kind of follow-up for this topic and deals about a little part of it. As with the previous topic, let's consider that our compiler has constexpr functions for std::initializer_list and std::array. Now, let's go straight to the…
Morwenn
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Why can't a placement-new expression be a constant expression?

According to [expr.const]/5.18: An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following: a new-expression ([expr.new]), unless…
Blackteahamburger
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__PRETTY_FUNCTION__ in constant expression

Please refer to this snippet: #include #include constexpr std::size_t strlen(char const* s) { std::size_t n = 0; while (*s++ != '\0') ++n; return n; } template struct X {}; int main()…
nicolai
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Why is this addition being silently ignored?

I have the following C code: #include #include int i; uint64_t a[] = { (uint64_t)&i, (uint64_t)&i + 0x8000000000000000 }; int main() { printf("%p %llx %llx\n", &i, a[0], a[1]); } If I compile this (as C or as C++) with…
user200783
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Endianness in constexpr

I want to create a constexpr function that returns the endianness of the system, like so: constexpr bool IsBigEndian() { constexpr int32_t one = 1; return (reinterpret_cast(one) == 0); } Now, since the function will get…
Rick de Water
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gcc and clang disagree on whether expression is constant evaluated

For the following program: struct S { int i; }; constexpr S f() { return std::is_constant_evaluated() ? S{1} : S{0}; } int main() { S s = f(); return s.i; } gcc returns 0, and clang returns 1. demo I don't think the evaluation of f is…
cigien
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Why are function addresses not constant expressions

Is there a way to use function addresses in constant expressions? void foo() {} int main() { static_assert(&foo, "test error"); } This won't compile. error C2057: expected constant expression The intention behind this is that I want to…
cooky451
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Why doesn't a Java constant divided by zero produce compile time error?

Possible Duplicate: Is 1/0 a legal Java expression? Why does this code compile? class Compiles { public final static int A = 7/0; public final static int B = 10*3; public static void main(String[] args) {} } If I take a look in the…
laitinen
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Is assert usable in constant expressions?

The assert-macro from provides a concise way of ensuring that a condition is met. If the argument evaluates to true, it shall not have any further effects. However, can its invocation also be used inside a constant expression in that case?
Columbo
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Java constant expressions and code elimination

As discussed here, javac and other Java compilers may provide code elimination capabilities for if-statements where the condition is a "Constant Expression". How is this affected if my code uses a constant expression that depends on other constant…
Markus A.
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Is gcc considering builtins of non-constant expression functions to be constant expressions

Please see the update for a better sample of the problem. The original code has a mix of issues which muddies the picture: This question Why can I call a non-constexpr function inside a constexpr function? presented the following code #include…
Shafik Yaghmour
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Can a null pointer constant be any integer constant expression evaluated to 0?

The standard says: "An integer constant expression with the value 0, or such an expression cast to type void*, is called a null pointer constant.67) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null…
RobertS supports Monica Cellio
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Is indexing a string literal an initializer constant expression?

The following code attempts to use array indexing on a string literal in two different constant contexts: static char x = "abcx"[3]; _Static_assert ("abcx"[3] == 'x', "..."); Judging by Compiler Explorer, there's clear consensus among tool vendors…
Alex Celeste
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Why is a constexpr function on a reference not constexpr?

Consider the following function: template auto concatenate(std::array &data1, std::array &data2) { std::array result; auto iter =…
Martijn Otto
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