assign struct with char array inside using "=" works?
arrays, c, char, struct
Solution
I see three explanations about the difference between pointers and arrays, but they don't seem to address the question being asked, as I understand it.
The question, as I understand it, is: "I know that I can't assign one array to another, so why can I do it if the array is inside a struct?"
The answer is, basically, "because the language said so". Even though a struct is an aggregate type, and may very well contain an array, the language spec says that structs are assignable, and therefore structs are assignable. This is section `6.5.16.1 Simple assignment` in both N1256 and N1570.
(Under the hood, the compiler may have to implement a memory copy, but that's the compiler's problem, not yours.)
Problem
I am reviewing someone's code and the developer has assigned one struct to another. The struct contains a char array. Somehow that "works", meaning the char array from struct A does get copied to struct B (not by reference). I'm totally baffled. Can s.o. explain that to me? I have written up a little program to illustrate the "problem". ``` #include <stdio.h> #include <string.h> /************************************** * some define and typedef for later **************************************/ #define MAX_LEN (80) typedef struct month_s { char name[MAX_LEN]; } month_st; /************************************** * This bit is clear. **************************************/ static void usingString() { char mCur[MAX_LEN] = {"Jan"}; char mNext[MAX_LEN] = {"Feb"}; /** this doesn't compile (of course) */ //mCur = mNext; /** we need a pointer/reference */ char *pmCur = &mCur[0]; /** however now we "copy by reference"...*/ pmCur = &(mNext[0]); /** ...so this change also applies to pmCur*/ strcpy(mNext, "Mar"); /** ...so pmCur is now "Mar"*/ printf("%s: %s\n", __func__, pmCur); } /************************************** * I though the same/s.th. similar * as above happens here. But this "works". * I'm surprised to see that not to be * the case. Can someone explain? **************************************/ static void usingStruct() { month_st mCur = {"Jan"}; month_st mNext = {"Feb"}; /** I would have thought this is "copy by reference" for the string! */ mCur = mNext; /** I would have thought this also overrides mCur.name 'cause it's pointing to mNext.name now */ strcpy(mNext.name, "Mar"); /** how come this works??? mCur.name is still "Feb"*/ printf("%s: %s\n", __func__, mCur.name); } /************************************** * just the main() **************************************/ int main() { usingString(); usingStruct(); return 0; } ```