C和C ++数组中的内存分配 [英] Memory allocation in C and C++ arrays

问题描述

C和C ++数组中的内存分配如何工作?

How does memory allocation in C and C++ arrays work?

推荐答案

数组索引只是进行加法的一种便捷且更有意义的方式.
因此，当您添加0来找到它时，任何数组(静态或动态)中的第一个元素都是0.

Array indices are just a convenient and more meaningful way of doing addition.
Hence the first element in any array, static or dynamic, is 0 as you add 0 to find it.

char *szDynamicStr = new char[12];
strcpy(szDynamicStr, "hello world");
printf("%c == %c\n", szDynamicStr[0], //This accesses the first element (''h'') as an array offset
	*(szDynamicStr + 0)); //This accesses the first element as an addition to the pointer value

szDynamicStr[0]和*(szDynamicStr + 0)都可以编译为完全相同的代码，因为数组会转换为指针偏移量.

指针指向数组中的第一个元素，因此偏移量0是第一个元素，偏移量1是第二个元素，依此类推...

如果这回答了您的问题，而您却不在乎它为什么起作用，请在此处停止阅读，否则您可能会感到困惑.

将数字添加到指针时，数字乘以编译器指向的数据大小(在这种情况下，单个ASCII字符为1个字节).

可以在代码中更好地解释:

Both szDynamicStr[0] and *(szDynamicStr + 0) compile to exactly the same code, because the array translates to the pointer offset.

The pointer is pointing to the first element in the array, so offset 0 is the first element, offset 1 is the 2nd element, etc...

If this answers your question and you don''t care why it works stop reading here or you might get confused.

When adding a number to a pointer, the number is multiplied by the size of the data that is getting pointed to (in this case 1 byte for a single ASCII char) by the compiler.

This is better explained in code:

DWORD pNumbers[] = { 0xD00D, 0xBADF00D }; //This is still a pointer, it is just easier to allocate this example this way.
printf("%u == %u\n", pNumbers[1], //The 2nd element in the array (0xBADF00D) as you would expect
	*(pNumbers + 1)); //This is also the 2nd element in the array. This actually adds 1 * sizeof(DWORD) = 4 bytes to the pointer in order to get the 2nd element in the array.

当内存分配因C ++的new运算符而失败时，它将引发异常.
当内存分配因C失败时，它将返回空指针.

使用new时，它将自动构造对象(称为对象的构造函数).
它的效率更高，但是用C分配内存会非常麻烦(尤其是对于不需要初始化/构造的数组).您可以绕过默认的构造函数，但是为了安全起见，需要添加一些记帐功能.

When a memory allocation fails with C++''s new operator, it throws an exception.
When a memory allocation fails with C, it returns a null pointer.

When you use new it automatically constructs the objects (the constructor of the object is called).
It''s more efficient, but a lot more cumbersome to allocate memory with C (especially if for arrays that you may not need to be initialized/constructed). You can bypass the default constructor, but add a bit of book-keeping for safety.

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