C++(C ?) 中变量的多次预递增操作
为什么下面的代码可以在 C++ 中编译？
int phew = 53;
++++++++++phew ;
同样的代码在 C 中失败，为什么？
Why does the following compile in C++?
int phew = 53;
++++++++++phew ;
The same code fails in C, why?
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注意：两个缺陷报告 DR#637和 DR#222 对于理解下面的行为理由。
作为解释，在 C++0x 中存在
值计算和副作用。例如，副作用是分配，而值计算是确定左值所指的内容或从左值中读取值。请注意，C++0x 不再具有序列点，并且这些内容的措辞是“之前序列”/“之后序列”。并且据说++v等价于v += 1，而v += 1等价于v = v + 1（只不过 v 仅计算一次）。这会产生++ (v = v + 1)，我将其写为inc = inc + 1，其中inc指的是左值v = v + 1的结果。在 C++0x 中，
++ ++v不是未定义的行为，因为对于a = b，赋值是在 b 和 a 的值计算之后、但在 a 的值计算之前排序的。赋值表达式。由此可见，v = v + 1中的赋值在inc的值计算之前排序。inc = inc + 1中的赋值是在inc的值计算之后排序的。最后，这两个分配都会被排序，并且不存在未定义的行为。Note: The two defect reports DR#637 and DR#222 are important to understand the below's behavior rationale.
For explanation, in C++0x there are
value computationsandside effects. A side effect for example is an assigment, and a value computation is determining what an lvalue refers to or reading the value out of an lvalue. Note that C++0x has no sequence points anymore and this stuff is worded in terms of "sequenced before" / "sequenced after". And it is stated that++vis equivalent tov += 1which is equivalent tov = v + 1(except that v is only evaluated once). This yields to++ (v = v + 1)which I will write asinc = inc + 1, whereincrefers to the lvalue result ofv = v + 1.In C++0x
++ ++vis not undefined behavior because fora = bthe assignment is sequenced after value computation of b and a, but before value computation of the assignment expression. It follows that the asignment inv = v + 1is sequenced before value computation ofinc. And the assignment ininc = inc + 1is sequenced after value computation ofinc. In the end, both assignments will thus be sequenced, and there is no undefined behavior.这是因为在
C++中，预自增运算符返回一个左值，并且它要求其操作数是一个左值。++++++++++phew ;解释为++(++(++(++(++phew))))但是你的代码调用
未定义行为，因为您试图在两个序列点。在
C中，预自增运算符返回一个右值，并要求其操作数是一个左值。所以你的代码不能在 C 模式下编译。That is because in
C++pre-increment operator returns anlvalueand it requires its operand to be anlvalue.++++++++++phew ;in interpreted as++(++(++(++(++phew))))However your code invokes
Undefined Behaviourbecause you are trying to modify the value ofphewmore than once between two sequence points.In
C, pre-increment operator returns anrvalueand requires its operand to be anlvalue. So your code doesn't compile in C mode.