c - arr[0] = arr[1] = C 中的值是不好的做法吗？

Question

正如我上面所说，这是不好的做法吗？在 ASM 中它会是什么样子？我的意思是，我不知道它是否被翻译成这样:

arr[0] = value;
arr[1] = value;

或者是这样的:

arr[1] = value;
arr[0] = arr[1];

第二个显然效率更低(imm vs mem)。

提前致谢。

Answer 1

根据 C 语法赋值运算符从右到左求值。

所以这个说法

arr[0] = arr[1] = value;

等价于

arr[0] = ( arr[1] = value );

来自 C 标准(6.5.16 赋值运算符)

3 An assignment operator stores a value in the object designated by the left operand. An assignment expression has the value of the left operand after the assignment,111) but is not an lvalue. The type of an assignment expression is the type the left operand would have after lvalue conversion. The side effect of updating the stored value of the left operand is sequenced after the value computations of the left and right operands. The evaluations of the operands are unsequenced.

所以根据报价你可以考虑这个说法

arr[0] = arr[1] = value;

喜欢

arr[1] = value;
arr[0] = arr[1];

至于效率，编译器可以生成与您想象的源代码不同的高效目标代码。