Menu
Search

API Documentation

version 2.084.0

overview

View source code
Display the source code in std/functional.d from which this page was generated on github.
Report a bug
If you spot a problem with this page, click here to create a Bugzilla issue.
Improve this page
Quickly fork, edit online, and submit a pull request for this page. Requires a signed-in GitHub account. This works well for small changes. If you'd like to make larger changes you may want to consider using local clone.

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.