問題描述

運(yùn)行下面的代碼顯示&x=ptr，那么x和*ptr怎么不相等?

running the following code shows that &x=ptr, so how come x and *ptr are not equal?

const int x=10;
int* ptr =(int*) &x;
*ptr = (*ptr)+1;

cout << &x << " " << x << "  " << ptr <<"  " <<*ptr;  //output : 0012FF60 10  0012FF60  11

推薦答案

C++ 實(shí)現(xiàn)只需要您遵守規(guī)則才能使程序運(yùn)行.你違反了規(guī)則.C++ 實(shí)現(xiàn)可能是這樣的:

The C++ implementation is only required to make a program work if you obey the rules. You violated the rules. The C++ implementation likely behaved this way:

• 因?yàn)?x 被聲明為 const，C++ 實(shí)現(xiàn)知道只要你遵守規(guī)則，它的值就不會(huì)改變.因此，無論在何處使用 x，C++ 實(shí)現(xiàn)都會(huì)使用 10，而無需費(fèi)心檢查 x 是否已更改.
• 因?yàn)?*ptr 指向一個(gè)非常量 int，所以實(shí)際執(zhí)行了存儲(chǔ)和讀取操作.這些工作"是因?yàn)樗赶虻膬?nèi)存(表示 x 的地方)實(shí)際上并未被操作系統(tǒng)標(biāo)記為只讀.因此，您可以進(jìn)行修改，盡管您不應(yīng)該這樣做.

• Because x is declared const, the C++ implementation knows its value cannot change as long as you obey the rules. So, wherever x is used, the C++ implementation uses 10 without bothering to check whether x has changed.
• Because *ptr points to a non-const int, stores to it and reads from it are actually performed. These "work" because the memory it points to (where x is represented) is not actually marked read-only by the operating system. Thus, you are able to make modifications in spite of the fact that you are not supposed to.

請注意，如果您遵守規(guī)則，C++ 實(shí)現(xiàn)的行為將可以工作.如果您沒有修改 x，那么在 x 出現(xiàn)的任何地方使用 10 都會(huì)正常工作.或者，如果您沒有將 x 聲明為 const，那么 C++ 實(shí)現(xiàn)不會(huì)假定它總是 10，因此每當(dāng) x 被訪問.這就是 C++ 標(biāo)準(zhǔn)對實(shí)現(xiàn)的所有要求:如果你遵守規(guī)則，它就可以工作.

Observe that the behavior of the C++ implementation would work if you obeyed the rules. If you had not modified x, then using 10 for x wherever it appeared would have worked normally. Or, if you had not declared x to be const, then the C++ implementation would not have assumed it would always be 10, so it would get the changed value whenever x was accessed. This is all the C++ standard requires of an implementation: That it work if you follow the rules.

當(dāng)您不遵守規(guī)則時(shí)，C++ 實(shí)現(xiàn)可能會(huì)以看似不一致的方式中斷.

When you do not follow the rules, a C++ implementation may break in seemingly inconsistent ways.

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