iammilind iammilind - 3 months ago 19
C++ Question

Is constexpr supported with lambda functions / expressions?

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 ?

Answer

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).

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