C++ 中的char * 对char * 运算时的“&&”是什么意思？

Question

I came across a code which extracted a token(c string) from a given string,but I am confused by the 5th line: tok && *tok . I just cannot figure out this operation's result. Does anybody happen to know?

const char* getfield(char* line, int num)
{
    const char* tok;
    for (tok = strtok(line, ";");
            tok && *tok;
            tok = strtok(NULL, ";\n"))
    {
        if (!--num)
            return tok;
    }
    return NULL;
}

Thank you!

Answer 1

它首先检查指针（ tok ）是否不是nullptr （指向nullptr ），然后如果指针不是nullptr ，它将检查它是否指向值（ *tok ）不是'\\0'内存。

Answer 2

This is the conditional expression, and it is checking that tok is not nullptr and that *tok , or the character, is not the "nul" character '\\0' .

In a C-style for-loop, you have the initalizer, the condition, and the increment expressions. The condition here is that tok evaluates to true, then *tok evaluates to true.

This can only occur if tok is not nullptr , and the character pointed to ( *tok ) is not '\\0' .

Answer 3

The meaning of && remains the same. It is still a logical operator. That statement is equivalent to

(tok != nullptr) && (*tok != '\0');

Answer 4

Can any point out what the below portion means?

{
    if (!--num)
        return tok;
}

I'm most interested in the if (!--num) . It's not making sense to me. In my head it reads as if NOT num = num - num. It's really cramping my brain.

I know, is the NOT operator. but the --num just doesn't compute in my brain.