问题描述
我最近碰到一个帖子，想知道是否输出
INT C = 0;
的printf（％D，C ++，C）;
也是不确定的？
我已经研究在得到一个分号修复后和preFIX运营商增值后才讲座。所以根据我来说，输出 0 是正确的！
Send your lecturer to me so that I can politely point out his mistake.
Exactly when the side effect of either pre- or postfix ++ and -- is applied is unspecified, apart from the requirement that it happen before the next sequence point. In an expression like
x = a++ * b
a may be updated immediately after a++ has been evaluated, or the update may be deferred until a++ * b has been evaluated and the result assigned to x, or anywhere in between.
This is why expressions like i++ * i++ and printf("%d %d", c++, c) and a[i++] = i and a host of others are all bad juju. You will get different results based on the compiler, optimization settings, surrounding code, etc. The language standard explicitly leaves the behavior undefined so that the compiler is under no obligation to "do the right thing", whatever the right thing may be. Remember, the definition for undefined behavior is
This is a deliberate design decision - the rationale for leaving the order of these operations unspecified is to give the implementation freedom to rearrange the evaluation order for optimization purposes. However, in exchange for this freedom, certain operations will not have well-defined results.
Note that a compiler is free to try to detect these cases and issue a diagnostic; printf("%d %d", c++, c); would be easy enough to catch, but this would be a bugger to detect in the general case. Imagine if that had been written printf("%d %d", (*p)++, c); if p points to c, then the behavior is undefined, otherwise it's okay. If p is assigned in a different translation unit, then there's no way to know at compile time whether this is a problem or not.
This concept is not difficult to understand, yet it is one of the most consistently misunderstood (and mis-taught) aspects of the C language. No doubt this is why the Java and C# language specifications force a specific evaluation order for everything (all operands are evaluated left-to-right, and all side effects are applied immediately).
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