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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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Dividing by zero in a constant expression

My toy compiler crashes if I divide by zero in a constant expression: int x = 1 / 0; Is this behaviour allowed by the C and/or C++ standards?
fredoverflow
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`static constexpr` function called in a constant expression is...an error?

I have the following code: class MyClass { static constexpr bool foo() { return true; } void bar() noexcept(foo()) { } }; I would expect that since foo() is a static constexpr function, and since it's defined before bar is declared, this…
Kyle Strand
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Why can't a constant pointer be a constant expression?

The following program compiles: template class Test{}; extern const int var = 42; //extern needed to force external linkage int main() { Test<&var> test; } This one, however, doesn't, which is a surprise for me: template…
Armen Tsirunyan
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int a=1, is a || 1 a constant expression?

N4527 5.20[expr.const]p5 A constant expression is either a glvalue core constant expression whose value refers to an entity that is a permitted result of a constant expression (as defined below), or a prvalue core constant expression whose …
stackcpp
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Why can creating a static const std::string cause an exception?

I have string constants, for strings that I use in multiple places in my app: namespace Common{ static const std::string mystring = "IamAwesum"; } When posting a question about something else (What happens to a .h file that is not included in a…
Rahul Iyer
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Using function argument as part of a constant expression - gcc vs clang

Consider the following code snippet: template struct B { }; template constexpr bool pred(T t) { return true; } template auto f(T t) -> decltype(B{}) { } clang++ (trunk) compiles the code g++ (trunk)…
Vittorio Romeo
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Is the comma operator allowed in a constant-expression in C++11?

In the process of answering this question on SO for C++11, I realized that in C++03 (as well as in C) the use of the comma operator is explicitly forbidden in a constant-expression. Paragraph 5.19/1 of the C++03 Standard on constant expressions…
Andy Prowl
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When does the 'const' qualifier guarantee the variable to be a constant expression?

As far as I know, the const qualifier in C++ basically declares internal linkage, and sometimes it allows the variable to be used as a constant expression so that it can be placed into array bounds, switch cases, etc. But apparently this is not…
morimn
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Why was the restriction on the comma operator being in a constant expression removed in C++11?

Recently when answering a question I realized that the comma operator is allowed in a constant expression in C++11 as long as the expression is surrounded by (), for example: int a[ (1, 2) ] ; Pre C++11 it is forbidden to use the comma operator in…
Shafik Yaghmour
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Pointer conversion issue with Ternary operator

I know the ternary operator has some surprising restrictions, but I was a bit baffled that this fails to compile for me: void foo(bool b) { int* ptr = ((b) ? NULL : NULL); } Obviously that's the minimum needed to show the problem. The error…
Roddy
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Why are lambda expressions not allowed in an unevaluated operands but allowed in the unevaluated portions of constant expressions?

If we look at the draft C++ standard section 5.1.2 Lambda expressions paragraph 2 says (emphasis mine going forward): The evaluation of a lambda-expression results in a prvalue temporary (12.2). This temporary is called the closure object. A…
Shafik Yaghmour
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error: switch quantity not an integer

I have researched my issue all over StackOverflow and multi-google links, and I am still confused. I figured the best thing for me is ask... Im creating a simple command line calculator. Here is my code so far: const std::string…
Ken
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Why "initializer element is not a constant" is... not working anymore?

static const int a = 42; static const int b = a; I would expect a compilation error in such code. The initializer must be a constant expression or a string literal. A value stored in an object with the type int with const type qualifier is not a…
KamilCuk
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Constant expression initializer for static class member of type double

In C++11 and C++14, why do I need constexpr in the following snippet: class Foo { static constexpr double X = 0.75; }; whereas this one produces a compiler error: class Foo { static const double X = 0.75; }; and (more surprisingly) this…
Stefano Sanfilippo
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Can the C# compiler or JIT optimize away a method call in a lambda expression?

I'm starting this question after a discussion which started (in comments) on another StackOverflow question, and I'm intrigued to know the answer. Considering the following expression: var objects = RequestObjects.Where(r => r.RequestDate >…
ChristopheD
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