Constant expressions

There are expressions in Ü, that are named constant expressions, which value may be calculated in compile-time. Such expressions may be used in some places where it is necessary - for array sizes, for template arguments, in constructions like static_assert and static_if.

An expression may be constant if:

• All variables inside it are constant
• Functions and operators in this expression are constexpr.

constexpr types

Variables inside constant expressions should be constant. Constant can be only variables of one of constexpr types.

constexpr types are:

• All fundamental types
• All enums
• All function pointers
• Arrays with constexpr element types
• Tuples with all element types constexpr
• Some struct types

constexpr structs requirements:

• All field types should be constexpr
• There should be no mutable reference fields
• There should be no explicit copy-constructors, copy-assignment operators, destructors

constexpr functions

Functions that may be used in constant expressions should be marked with constexpr keyword. Such functions should have a body, constexpr for prototype declarations is not allowed.

constexpr function requirements:

• Parameter types and return types should be constexpr, but not function pointers
• They can’t be marked as unsafe
• They should not contain unsafe blocks
• They should not do reference pollution (see corresponding chapter)
• They can’t contain definitions of variables of non-constexpr types
• They can’t contain calls to non-constexpr functions
• They can’t call functions via pointers

constexpr functions may have mutable reference parameters, but such functions can’t be called with constexpr arguments in order to produce constexpr result. Almost all constructions are allowed inside constexpr functions (except some forbidden ones) this includes conditions, loops, halt, etc.

constexpr function example:

fn constexpr ArrSumm( [ u32, 16 ]& arr ) : u32
{
    auto mut res= 0u;
    auto mut i= 0s;
    while( i < 16s )
    {
        res+= arr[i];
        ++i;
    }
    return res;
}

constexpr variables

Immutable variables of constexpr type which are constantly-initialized are considered to be constexpr. If it’s necessary to ensure that a variable is constexpr, constexpr mutability modifier should be used.

auto x= 0; // Variable is immutable, its initializer is constant, thus variable is constexpr
auto imut y= x + 5; // Same as above, but variable is explicitly marked as immutable
var i32 z(66), imut v(-5), w= y / 2; // All these variables are constexpr
var [ i32, 2 ] arr[ 0 + z, w * 10 ]; // These are constexpr too
auto constexpr x_copy= x; // Explicitly specify constexpr
var i32 constexpr ensure_constant(y); // Explicitly specify constexpr

var i32 mut nonconst= 0; // Initializer is constant, but the variable itself isn't because it is mutable
auto constexpr wtf= nonconst; // An error will be generated - initializer of constexpr variable is not constexpr