Is constexpr supported with lambda functions / expressions?

65
votes
struct Test
{
  static const int value = []() -> int { return 0; } ();
};

With gcc-4.6 I get something like, error: function needs to be constexpr. I have tried multiple combinations of putting constexpr at various places, but no luck.

Is constexpr supported for lambda functions as well (irrespective of return type specified or not) ? What is the correct syntax ?

Any work around possible ?

4
c++c++11lambdaconstexpr
Under what circumstances would a constexpr lambda function be useful? It seems to me like it would only end up adding another pair of braces around the actual expression – bdonlan
@bdonlan, I have a use case for that (to calculate number of __VA_ARGS__ in macro). But explaining that will be a whole new question. – iammilind
Also see Why are lambda expressions not allowed in an unevaluated operands but allowed in the unevaluated portions of constant expressions? – Shafik Yaghmour

4 Answers

36
votes

Update: As of C++17, lambdas are permitted in constant expressions.

Lambdas are currently (C++14) not allowed in constant expressions as per [expr.const]/(2.6), but they will once N4487 is accepted (which can be found in the working draft N4582):

This proposal suggests allowing lambda-expressions in constant expressions, removing an existing restriction. The authors propose that certain lambda-expressions and operations on certain closure objects be allowed to appear within constant expressions. In doing so, we also propose that a closure type be considered a literal type if the type of each of its data-members is a literal type; and, that if the constexpr specifier is omitted within the lambda-declarator, that the generated function call operator be constexpr if it would satisfy the requirements of a constexpr function (similar to the constexpr inference that already occurs for implicitly defined constructors and the assignment operator functions).

27
votes

From the C++0x FDIS §7.1.5[dcl.constexpr]/1:

The constexpr specifier shall be applied only to the definition of a variable, the declaration of a function or function template, or the declaration of a static data member of a literal type.

A lambda expression is none of those things and thus may not be declared constexpr.

14
votes

Prior to C++17 lambdas are not compatible with constexpr. They cannot be used inside constant expressions.

Starting with C++17 lambdas are constexpr where it makes sense. The proposal N4487 will be put into the C++17 standard. On his website Herb Sutter, chair of the ISO C++ committee, stated the following:

Lambdas are now allowed inside constexpr functions.

8
votes

FFWD to year 2018 :)

auto my_const_expression_lambda = []()
  constexpr -> bool
{
   return true ;
}

Since c++17