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Alias std.functional.forward

Forwards function arguments while keeping out, ref, and lazy on the parameters.

alias forward(args...) = fwd;

Parameters

NameDescription
args a parameter list or an AliasSeq.

Returns

An AliasSeq of args with out, ref, and lazy saved.

Example

class C
{
    static int foo(int n) { return 1; }
    static int foo(ref int n) { return 2; }
}

// with forward
int bar()(auto ref int x) { return C.foo(forward!x); }

// without forward
int baz()(auto ref int x) { return C.foo(x); }

int i;
writeln(bar(1)); // 1
writeln(bar(i)); // 2

writeln(baz(1)); // 2
writeln(baz(i)); // 2

Example

void foo(int n, ref string s) { s = null; foreach (i; 0 .. n) s ~= "Hello"; }

// forwards all arguments which are bound to parameter tuple
void bar(Args...)(auto ref Args args) { return foo(forward!args); }

// forwards all arguments with swapping order
void baz(Args...)(auto ref Args args) { return foo(forward!args[$/2..$], forward!args[0..$/2]); }

string s;
bar(1, s);
writeln(s); // "Hello"
baz(s, 2);
writeln(s); // "HelloHello"

Example

struct X {
    int i;
    this(this)
    {
        ++i;
    }
}

struct Y
{
    private X x_;
    this()(auto ref X x)
    {
        x_ = forward!x;
    }
}

struct Z
{
    private const X x_;
    this()(auto ref X x)
    {
        x_ = forward!x;
    }
    this()(auto const ref X x)
    {
        x_ = forward!x;
    }
}

X x;
const X cx;
auto constX = (){ const X x; return x; };
static assert(__traits(compiles, { Y y = x; }));
static assert(__traits(compiles, { Y y = X(); }));
static assert(!__traits(compiles, { Y y = cx; }));
static assert(!__traits(compiles, { Y y = constX(); }));
static assert(__traits(compiles, { Z z = x; }));
static assert(__traits(compiles, { Z z = X(); }));
static assert(__traits(compiles, { Z z = cx; }));
static assert(__traits(compiles, { Z z = constX(); }));


Y y1 = x;
// ref lvalue, copy
writeln(y1.x_.i); // 1
Y y2 = X();
// rvalue, move
writeln(y2.x_.i); // 0

Z z1 = x;
// ref lvalue, copy
writeln(z1.x_.i); // 1
Z z2 = X();
// rvalue, move
writeln(z2.x_.i); // 0
Z z3 = cx;
// ref const lvalue, copy
writeln(z3.x_.i); // 1
Z z4 = constX();
// const rvalue, copy
writeln(z4.x_.i); // 1

Authors

Andrei Alexandrescu

License

Boost License 1.0.