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std.traits.Select/select - multiple declarations

Function select

Select one of two functions to run via template parameter.

A select(bool cond : true, A, B) (
  A a,
  lazy B b
);

B select(bool cond : false, A, B) (
  lazy A a,
  B b
);

Parameters

NameDescription
cond A bool which determines which function is run
a The first function
b The second function

Returns

a without evaluating b if cond is true. Otherwise, returns b without evaluating a.

Example

real run() { return 0; }
int fail() { assert(0); }
auto a = select!true(run(), fail());
auto b = select!false(fail(), run());
static assert(is(typeof(a) == real));
static assert(is(typeof(b) == real));

Alias Select

Aliases itself to T[0] if the boolean condition is true and to T[1] otherwise.

alias Select(bool condition, T...) = Alias!(T[!condition]);

Example

// can select types
static assert(is(Select!(true, int, long) == int));
static assert(is(Select!(false, int, long) == long));
static struct Foo {}
static assert(is(Select!(false, const(int), const(Foo)) == const(Foo)));

// can select symbols
int a = 1;
int b = 2;
alias selA = Select!(true, a, b);
alias selB = Select!(false, a, b);
writeln(selA); // 1
writeln(selB); // 2

// can select (compile-time) expressions
enum val = Select!(false, -4, 9 - 6);
static assert(val == 3);
}

/**
Select one of two functions to run via template parameter.

Params:
cond = A `bool` which determines which function is run
a = The first function
b = The second function

Returns:
`a` without evaluating `b` if `cond` is `true`.
Otherwise, returns `b` without evaluating `a`.
*/
A select(bool cond : true, A, B)(A a, lazy B b) { return a;

Authors

Walter Bright, Tomasz Stachowiak (isExpressions), Andrei Alexandrescu, Shin Fujishiro, Robert Clipsham, David Nadlinger, Kenji Hara, Shoichi Kato

License

Boost License 1.0.