C++ 入门第 5 版 : Stream iterators and Sales_item

Question

在 C++ primer 5Ed 中，该章讨论了流迭代器，他给出了这个示例(第 512 页):

int main(){

    istream_iterator<Sales_item> item_iter(cin), eof;
    ostream_iterator<Sales_item> out_iter(cout, "\n");
    // store the first transaction in sum and read the next record
    Sales_item sum = *item_iter++;
    while (item_iter != eof) {
        // if the current transaction (which is stored in item_iter) has the same ISBN
        if (item_iter->isbn() == sum.isbn())
            sum += *item_iter++; // add it to sum and read the next
        transaction
        else {
            out_iter = sum; // write the current sum
            sum = *item_iter++; // read the next transaction
        }
    }
    out_iter = sum; // remember to print the last set of records

}

它工作正常，但我注意到在循环内它在 if-statement 中做了两次相同的事情:

if (item_iter->isbn() == sum.isbn())
    sum += *item_iter++; // add it to sum and read the next transaction
else {
    out_iter = sum; // write the current sum
    sum = *item_iter++; // read the next transaction
}

为什么他不应该将它优化为只在一条语句中使用解除引用和递增？所以只有当记录不同时打印，无论条件如何递增和取消引用:

while (item_iter != eof) {
    if (item_iter->isbn() != sum.isbn()) // instead of check for equality
        out_iter = sum; // write the current sum
    sum = *item_iter++; // read the next transaction
}

这是个好主意还是我应该坚持书中的例子？谢谢？

Answer 1

请坚持书中的例子。您没有准确检查语句。

在

if (item_iter->isbn() == sum.isbn())
    sum += *item_iter++; // add it to sum and read the next transaction
else {
    out_iter = sum; // write the current sum
    sum = *item_iter++; // read the next transaction
}

说法不同

请参阅:sum += 和 sum =

也许你误解了。

没什么大不了的。