Programming in D: The Ultimate Guide for Software Engineers/

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Lvalues and Rvalues

Lvalues and Rvalues

This lesson explains the difference between the lvalues and rvalues, limitations of rvalues and how both can be passed as parameters to functions.

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The difference between lvalues and rvalues #

The value of every expression is classified as either an lvalue or an rvalue. A simple way of differentiating the two is thinking of lvalues as actual variables (including elements of arrays and associative arrays) and rvalues as temporary results of expressions (including literals).
As a demonstration, the first writeln() expression below uses only values, and the other one uses only rvalues:

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import std.stdio;
void main() {
    int i;
    immutable(int) imm;
    auto arr = [ 1 ];
    auto aa = [ 10 : "ten" ];
    /* All of the following arguments are lvalues. */
    writeln(i,          // mutable variable
            imm,        // immutable variable
            arr,        // array
            arr[0],     // array element
            aa[10]);    // associative array element
                        // etc.
    enum message = "hello";
    /* All of the following arguments are rvalues. */
    writeln(42,               // a literal
            message,          // a manifest constant
            i + 1,            // a temporary value
            calculate(i));    // return value of a function
                              // etc.
}
int calculate(int i) {
    return i * 2;
}
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