C语言 C - 初始化结构数组

C - initialize array of structs

提问by pmg

I am having a problem initializing an array of structs. I'm not sure if I am doing it right because I get "initialization from incompatible pointer type" & "assignment from incompatible pointer type". I added in the code where I get these warnings, and when I try to print the data from the struct I just get garbage such as @@###

我在初始化结构数组时遇到问题。我不确定我是否做得对，因为我得到了“从不兼容的指针类型初始化”和“从不兼容的指针类型赋值”。我在代码中添加了这些警告，当我尝试从结构中打印数据时，我只会得到垃圾，例如@@###

typedef struct
{
    char* firstName;
    char* lastName;
    int day;
    int month;
    int year;

}student;

//initialize array

//初始化数组

    student** students = malloc(sizeof(student));
    int x;
    for(x = 0; x < numStudents; x++)
    {
        //here I get: "assignment from incompatible pointer type" 
        students[x] = (struct student*)malloc(sizeof(student));
    }

    int arrayIndex = 0;

//add struct

//添加结构体

 //create student struct
        //here I get: "initialization from incompatible pointer type"
        student* newStudent = {"john", "smith", 1, 12, 1983};

        //add it to the array
        students[arrayIndex] = newStudent;
        arrayIndex++;

回答by pmg

This is incorrect:

这是不正确的：

student** students = malloc(sizeof(student));

You do not want a **. You want a *and enough space for how ever many students you need

你不想要一个**. 您需要*足够的空间来容纳您需要的学生数量

student *students = malloc(numStudents * sizeof *students); // or sizeof (student)
for (x = 0; x < numStudents; x++)
{
    students[x].firstName = "John"; /* or malloc and strcpy */
    students[x].lastName = "Smith"; /* or malloc and strcpy */
    students[x].day = 1;
    students[x].month = 12;
    students[x].year = 1983;
}

If you still want to use the code in your "//add struct" section, you'll need to change the line:

如果您仍想使用“//add struct”部分中的代码，则需要更改该行：

student* newStudent = {"john", "smith", 1, 12, 1983};

to

到

student newStudent = {"john", "smith", 1, 12, 1983};

You were getting "initialization from incompatible pointer type" because you were attempting to initialize a pointer to studentwith an object of type student.

您正在“从不兼容的指针类型初始化”，因为您试图student用类型为的对象初始化指向的指针student。

回答by Tommy

Unrelated to the compiler warnings, but your initial malloc is wrong; you want:

与编译器警告无关，但您的初始 malloc 是错误的；你要：

malloc(sizeof(student *)* numStudents)

To allocate room for a total of 'numStudents' pointers to a student. The line:

为一个学生的总共 'numStudents' 指针分配空间。线路：

students[x] = (struct student*)malloc(sizeof(student));

Should be:

应该：

students[x] = (student*)malloc(sizeof(student));

There's no such thing as 'struct student'. You've declared an unnamed struct and typedef'd it to 'student'. Compare and contrast with:

没有“结构学生”这样的东西。您已经声明了一个未命名的结构并将其 typedef 为 'student'。比较和对比：

struct student
{
    char* firstName;
    char* lastName;
    int day;
    int month;
    int year;

};

Which would create a 'struct student' type but require you (in C) to explicitly refer to struct student rather than merely student elsewhere. This rule is changed for C++, so your compiler may be a bit fuzzy about it.

这将创建一个“struct student”类型，但要求您（在 C 中）明确引用 struct student 而不仅仅是其他地方的 student 。此规则已针对 C++ 更改，因此您的编译器可能对此有点模糊。

As for:

至于：

student* newStudent = {"john", "smith", 1, 12, 1983};

That should be:

那应该是：

student newStudent = {"john", "smith", 1, 12, 1983};

As the curly brace syntax is a direct literal, not something somewhere else that you need to point to.

由于花括号语法是直接文字，而不是您需要指向的其他地方。

EDIT: on reflection, I think aaa may have taken more of an overview of this than I have. Is it possible that you're inadvertently using an extra level of pointer dereference everywhere? So you'd want:

编辑：经过反思，我认为 aaa 可能比我更了解这一点。您是否可能无意中在任何地方都使用了额外级别的指针取消引用？所以你会想要：

student* students = malloc(sizeof(student) * numStudents);

/* no need for this stuff: */
/*int x;
for(x = 0; x < numStudents; x++)
{
    //here I get: "assignment from incompatible pointer type" 
    students[x] = (struct student*)malloc(sizeof(student));
}*/

int arrayIndex = 0;

And:

和：

student newStudent = {"john", "smith", 1, 12, 1983};

//add it to the array
students[arrayIndex] = newStudent;
arrayIndex++;

Subject the array not being used outside of scope of newStudent. Otherwise copying the pointers to strings is incorrect.

主题未在 newStudent 范围之外使用的数组。否则将指针复制到字符串是不正确的。

回答by Anycorn

student* students = malloc(sizeof(student)*numStudents);
int x;
for(x = 0; x < numStudents; x++)
{
    student newStudent = {"john", "smith", 1, 12, 1983}; // string copy are wrong still
    students[x] = newStudent;
}

回答by frarees

When initializing, shoudn't it be like this?

初始化的时候，不应该是这样吗？

student** students = (struct student**)malloc(sizeof(student*)*numStudents);

However, why pointer to a pointer? Just with a pointer to struct is enough I think.

但是，为什么要指向指针呢？我认为只要有一个指向 struct 的指针就足够了。