c - : '=' : left operand must be l-value? 是什么意思

Question

我正在使用结构编写代码，该结构将读取包含书籍数据的文本文件。姓名、作者、出版商、流派等...

我的头文件如下所示:

typedef struct book{    

    char name[NAME_LENGTH];
    char authors[AUTHORS_NAME_LENGTH];
    char publisher[PUBLISHER_NAME_LENGTH];
    char genre[GENRE_LENGTH];
    int year;
    int num_pages;
    int copies;
}book;


typedef struct library
{    

    book books[BOOK_NUM];

}library;

typedef char* string;

代码是:

    if (NULL == (incoming_books = fopen(".\books.txt", "r")))
    {            /* opening file for reading */
        /*printf("Error opening file"); write to file*/
        exit(1);
    }
    while (!feof(incoming_books))
    {
        fgets(line,200,incoming_books);//copies one line, assuming no longer than 200 chars

        idx_helper = strchr(line, '$');//finds '$' index, attribures are seperated by "$$$"
        index = (int)(idx_helper - line);// cast index into int
        char_num = index;
        if (NULL != memcpy(temp_string, line, char_num))//copies string (name)
            temp_book->name = *temp_string;

        index += 3;                         // incrementing index by 3
        idx_helper = strchr(&line[index], '$');   // same for authors
        index = (int)(idx_helper - &line[index]);
        char_num = index;
        if (NULL != memcpy(temp_string, &line[index], char_num*sizeof(char)))
        temp_book->authors = *temp_string;
    }

每个图书属性依此类推

我收到两个错误: 1. 错误 7 错误 C2106: '=' : 左操作数必须是左值 它指向行 temp_book->name = *temp_string;和 temp_book->authors = *temp_string; 2. IntelliSense:表达式必须是可修改的左值 它指向相同的线。

会不会是指针问题？

Answer 1

因为数组是不可修改的左值。

摘自C11 Standard Draft N1570 —(此答案中突出显示了粗体部分，以帮助读者找到有趣的部分)。

6.3.2.1 Lvalues, arrays, and function designators

1. An lvalue is an expression (with an object type other than void) that potentially designates an object; ⁶⁴⁾ if an lvalue does not designate an object when it is evaluated, the behavior is undefined. When an object is said to have a particular type, the type is specified by the lvalue used to designate the object. A modifiable lvalue is an lvalue that does not have array type, does not have an incomplete type, does not have a const-qualified type, and if it is a structure or union, does not have any member (including, recursively, any member or element of all contained aggregates or unions) with a const-qualified type.

---

⁶⁴⁾_{The name ‘‘lvalue’’ comes originally from the assignment expression E1 = E2, in which the left operand E1 is required to be a (modifiable) lvalue. It is perhaps better considered as representing an object ‘‘locator value’’. What is sometimes called ‘‘rvalue’’ is in this International Standard described as the ‘‘value of an expression’’.}

您使用memcpy()复制到temp_string，您应该使用它直接复制到数组。另外，不要使用 sizeof(char)，因为它永远不会是 1 之外的任何内容，并且始终为 nul 终止符分配空间，然后复制nul 终止符。否则，当您将数组传递给需要 字符串 的函数时，例如各种 str* 函数或带有 的 printf() ” %s" 说明符未定义的行为将发生。

此外，在您的代码中有这个

while (!feof(incoming_books))

只有当 fgets() 失败时，这才会是true，那么它会在第一次 fgets() 失败后发生，并且当您永远不要检查最后一行是否会重复，你应该这样做

while (fgets(line, 200, incoming_books) != NULL)