Struct within a struct in C++

Question

How do you access members of a struct that is defined within another struct?
Suppose we have a struct defined as:

struct FR {
    size_t n;
    struct FR_1{
        unsigned char r;
        unsigned char g;
        unsigned char b;
    };
};

Under visual studio 2015,writing:

    struct FR x;
    x.FR_1.

does not display options for FR_1 members.On the other hand,writing:

struct FR_1 y; Says: Error,incomplete type is not allowed.

How do you deal with this kind of struct?

You know that's covered in every [decent C++ book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list?lq=1). Maybe you should get one and learn the language. — Rob K, May 24 '16 at 21:30

AlexD · Accepted Answer · 2016-05-24T21:26:17.767

11

The sample declares the type struct FR_1, not a member of that type. Instead, try:

struct FR {
    size_t n;
    struct FR_1 {
        unsigned char r;
        unsigned char g;
        unsigned char b;
    } fr1;
};

FR x;
x.fr1.r = 0;

edited May 24 '16 at 21:26

answered May 24 '16 at 21:19

AlexD

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score 5 · Answer 2 · answered May 24 '16 at 21:22

struct FR {
    size_t n; // < Declaration of member variable
    struct FR_1{ // < Declaration of nested type
        unsigned char r;
        unsigned char g;
        unsigned char b;
    };
    FR_1 fr1; // < Declaration of member variable
};

You need to declare a variable of the type FR_1 in your FR structrure, not only the type itself.

FR fr;
fr.fr1.r = 0;

score 0 · Answer 3 · answered May 24 '16 at 21:33

You need to actually create an instance of the structure. A normal struct declaration follows the form

struct struct-name {
    members
} inst;

So you need to declare it as

struct FR {
    size_t n;

    struct FR_1 {
        unsigned char r;
        unsigned char g;
        unsigned char b;
    } fr1;
};

Now you can write

FR fr;
fr.fr1.r = 255;
. . .

Struct within a struct in C++

3 Answers3