在C / C ++中获取结构或联合成员的字节大小或字符大小？

Question

Let's say that I want to get the size in bytes or in chars for the name field from:

struct record
{
    int id;
    TCHAR name [50];
};

sizeof(record.name) does not work.

Answer 1

The solution for this is not so pretty as you may think:

size_in_byte = sizeof(((struct record *) 0)->name)

size_in_chars = _countof(((struct record *) 0)->name)

If you want to use the second one on other platforms than Windows try:

#define _countof(array) (sizeof(array)/sizeof(array[0]))

Answer 2

If you create an instance first, it will work.

record r;
sizeof(r.name);

Answer 3

In C++:

#include <iostream>
using namespace std;;

struct record
{
    int id;
    char name [50];
};

int main() {
    cout << sizeof( record::name) << endl;
}

Edit: A couple of people have pointed out that this is C++0x code, so I guess I must retract my unkind comment regarding VC++. This is not a programming construct I have ever used in my own C++ code, but I have to wonder why sizeof would not work this way in C++03? You hand it a name and it gives you the size. I'd have thought it would take some effort for it not to work. But such is the wonder of the C++ Standard :-)

Answer 4

record is the name of a type, but record.name is not. You somehow have to access name through an instance of the struct. Sorin's answer is the usual C solution:

sizeof ((struct record*)0)->name;

This creates a pseudo-pointer to an instance (or pointer to a pseudo-instance) of struct record , then access the name member, and pass that expression to sizeof . It works because sizeof doesn't attempt to evaluate the pointer expression, it just uses it to compute the size.

Answer 5

你可能想读这个 ，因为它讨论了同样的问题并提供了这个线程中提到的所有选项，还有更多。

Answer 6

struct record
{
    static const int kMaxNameChars=50;
    int id;
    TCHAR name [kMaxNameChars];
};


sizeof(TCHAR)*record::kMaxNameChars //"sizeof(record.name)"
//record::kMaxNameChars sufficient for many purposes.

Portable, perfectly safe and IMO being explicit about raw array length is good practice.

(edit: you might have to macro it in C, if the compiler gets upset about variable array lengths. if you do, consider defining a static const int to the value of the macro anyway!)