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version 2.090.0

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Template std.typecons.Proxy

Creates a proxy for the value a that will forward all operations while disabling implicit conversions. The aliased item a must be an lvalue. This is useful for creating a new type from the "base" type (though this is not a subtype-supertype relationship; the new type is not related to the old type in any way, by design).

template Proxy(alias a) ;

The new type supports all operations that the underlying type does, including all operators such as +, --, <, [], etc.

Parameters

NameDescription
a The value to act as a proxy for all operations. It must be an lvalue.

Example

struct MyInt
{
    private int value;
    mixin Proxy!value;

    this(int n){ value = n; }
}

MyInt n = 10;

// Enable operations that original type has.
++n;
writeln(n); // 11
writeln(n * 2); // 22

void func(int n) { }

// Disable implicit conversions to original type.
//int x = n;
//func(n);

Example

The proxied value must be an lvalue. 

struct NewIntType
{
    //Won't work; the literal '1'
    //is an rvalue, not an lvalue
    //mixin Proxy!1;

    //Okay, n is an lvalue
    int n;
    mixin Proxy!n;

    this(int n) { this.n = n; }
}

NewIntType nit = 0;
nit++;
writeln(nit); // 1


struct NewObjectType
{
    Object obj;
    //Ok, obj is an lvalue
    mixin Proxy!obj;

    this (Object o) { obj = o; }
}

NewObjectType not = new Object();
assert(__traits(compiles, not.toHash()));

Example

There is one exception to the fact that the new type is not related to the old type. Pseudo-member functions are usable with the new type; they will be forwarded on to the proxied value. 

import std.math;

float f = 1.0;
assert(!f.isInfinity);

struct NewFloat
{
    float _;
    mixin Proxy!_;

    this(float f) { _ = f; }
}

NewFloat nf = 1.0f;
assert(!nf.isInfinity);

Authors

Andrei Alexandrescu, Bartosz Milewski, Don Clugston, Shin Fujishiro, Kenji Hara

License

Boost License 1.0.