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Simple assignment expressionsСодержание книги
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For an expression expr of the form w = expr-rhs: · The definite assignment state of v before expr-rhs is the same as the definite assignment state of v before expr. · If w is the same variable as v, then the definite assignment state of v after expr is definitely assigned. Otherwise, the definite assignment state of v after expr is the same as the definite assignment state of v after expr-rhs. 5.3.3.24 && expressions For an expression expr of the form expr-first && expr-second: · The definite assignment state of v before expr-first is the same as the definite assignment state of v before expr. · The definite assignment state of v before expr-second is definitely assigned if the state of v after expr-first is either definitely assigned or “definitely assigned after true expression”. Otherwise, it is not definitely assigned. · The definite assignment state of v after expr is determined by: o If expr-first is a constant expression with the value false, then the definite assignment state of v after expr is the same as the definite assignment state of v after expr-first. o Otherwise, if the state of v after expr-first is definitely assigned, then the state of v after expr is definitely assigned. o Otherwise, if the state of v after expr-second is definitely assigned, and the state of v after expr-first is “definitely assigned after false expression”, then the state of v after expr is definitely assigned. o Otherwise, if the state of v after expr-second is definitely assigned or “definitely assigned after true expression”, then the state of v after expr is “definitely assigned after true expression”. o Otherwise, if the state of v after expr-first is “definitely assigned after false expression”, and the state of v after expr-second is “definitely assigned after false expression”, then the state of v after expr is “definitely assigned after false expression”. o Otherwise, the state of v after expr is not definitely assigned. In the example class A the variable i is considered definitely assigned in one of the embedded statements of an if statement but not in the other. In the if statement in method F, the variable i is definitely assigned in the first embedded statement because execution of the expression (i = y) always precedes execution of this embedded statement. In contrast, the variable i is not definitely assigned in the second embedded statement, since x >= 0 might have tested false, resulting in the variable i being unassigned. 5.3.3.25 || expressions For an expression expr of the form expr-first || expr-second: · The definite assignment state of v before expr-first is the same as the definite assignment state of v before expr. · The definite assignment state of v before expr-second is definitely assigned if the state of v after expr-first is either definitely assigned or “definitely assigned after false expression”. Otherwise, it is not definitely assigned. · The definite assignment statement of v after expr is determined by: o If expr-first is a constant expression with the value true, then the definite assignment state of v after expr is the same as the definite assignment state of v after expr-first. o Otherwise, if the state of v after expr-first is definitely assigned, then the state of v after expr is definitely assigned. o Otherwise, if the state of v after expr-second is definitely assigned, and the state of v after expr-first is “definitely assigned after true expression”, then the state of v after expr is definitely assigned. o Otherwise, if the state of v after expr-second is definitely assigned or “definitely assigned after false expression”, then the state of v after expr is “definitely assigned after false expression”. o Otherwise, if the state of v after expr-first is “definitely assigned after true expression”, and the state of v after expr-second is “definitely assigned after true expression”, then the state of v after expr is “definitely assigned after true expression”. o Otherwise, the state of v after expr is not definitely assigned. In the example class A the variable i is considered definitely assigned in one of the embedded statements of an if statement but not in the other. In the if statement in method G, the variable i is definitely assigned in the second embedded statement because execution of the expression (i = y) always precedes execution of this embedded statement. In contrast, the variable i is not definitely assigned in the first embedded statement, since x >= 0 might have tested true, resulting in the variable i being unassigned. 5.3.3.26! expressions For an expression expr of the form! expr-operand: · The definite assignment state of v before expr-operand is the same as the definite assignment state of v before expr. · The definite assignment state of v after expr is determined by: o If the state of v after expr-operand is definitely assigned, then the state of v after expr is definitely assigned. o If the state of v after expr-operand is not definitely assigned, then the state of v after expr is not definitely assigned. o If the state of v after expr-operand is “definitely assigned after false expression”, then the state of v after expr is “definitely assigned after true expression”. o If the state of v after expr-operand is “definitely assigned after true expression”, then the state of v after expr is “definitely assigned after false expression”. Expressions For an expression expr of the form expr-first?? expr-second: · The definite assignment state of v before expr-first is the same as the definite assignment state of v before expr. · The definite assignment state of v before expr-second is the same as the definite assignment state of v after expr-first. · The definite assignment statement of v after expr is determined by: o If expr-first is a constant expression (§7.19) with value null, then the the state of v after expr is the same as the state of v after expr-second. · Otherwise, the state of v after expr is the same as the definite assignment state of v after expr-first. Expressions For an expression expr of the form expr-cond? expr-true: expr-false: · The definite assignment state of v before expr-cond is the same as the state of v before expr. · The definite assignment state of v before expr-true is definitely assigned if and only if one of the following holds: o expr-cond is a constant expression with the value false o the state of v after expr-cond is definitely assigned or “definitely assigned after true expression”. · The definite assignment state of v before expr-false is definitely assigned if and only if one of the following holds: o expr-cond is a constant expression with the value true · the state of v after expr-cond is definitely assigned or “definitely assigned after false expression”. · The definite assignment state of v after expr is determined by: o If expr-cond is a constant expression (§7.19) with value true then the state of v after expr is the same as the state of v after expr-true. o Otherwise, if expr-cond is a constant expression (§7.19) with value false then the state of v after expr is the same as the state of v after expr-false. o Otherwise, if the state of v after expr-true is definitely assigned and the state of v after expr-false is definitely assigned, then the state of v after expr is definitely assigned. o Otherwise, the state of v after expr is not definitely assigned. Anonymous functions For a lambda-expression or anonymous-method-expression expr with a body (either block or expression) body: · The definite assignment state of an outer variable v before body is the same as the state of v before expr. That is, definite assignment state of outer variables is inherited from the context of the anonymous function. · The definite assignment state of an outer variable v after expr is the same as the state of v before expr. The example delegate bool Filter(int i); void F() { // Error, max is not definitely assigned max = 5; generates a compile-time error since max is not definitely assigned where the anonymous function is declared. The example delegate void D(); void F() { d(); // Error, n is not definitely assigned also generates a compile-time error since the assignment to n in the anonymous function has no affect on the definite assignment state of n outside the anonymous function.
Variable references A variable-reference is an expression that is classified as a variable. A variable-reference denotes a storage location that can be accessed both to fetch the current value and to store a new value. variable-reference: In C and C++, a variable-reference is known as an lvalue.
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