Problems with struct - c++

Question

so i was having two types of issues from my code... the general forms are:
"left of ... must have class/struct/union " :
doc.id[j] = NULL;
doc.ISBN[j] = NULL;
doc.title[j] = NULL;
doc.year[j] = NULL;
and " 'doc' uses undefined struct 'main::m_doc'"
struct m_doc doc;
while the struct m_doc doc is included in the top of each function .
here is my struct :
struct m_doc{
char id[30];
int ISBN[30];
char title[50];
char author[50];
int year[30];
};
my fist function :
void menudoc()
{ struct m_doc doc;
int c = 0, a = 0, b;
cout << "1. Them tai lieu moi" << endl;
cout << "2. Xoa tai lieu" << endl;
cout << "3. Bao cao theo nam xuat ban" << endl;
cout << "0. Tro lai menu truoc" << endl;
cout << "Ban chon:";
cin >> b;
switch (b)
{
case 1:
{
for (int i = 0;; i++)
{
if (doc.id[i] == NULL && doc.ISBN[i] == NULL && doc.title[i] == NULL && doc.author[i] == NULL && doc.year[i] == NULL)
{
i = c; break;
}
else continue;
}
cin >> doc.author[c] ;
cin >> doc.id[c] ;
cin >> doc.ISBN[c] ;
cin >> doc.title[c];
cin >> doc.year[c];
for (int i = 0;; i++){
for (int j = 1;; j++){
if (doc.id[i] == doc.id[j]){
doc.author[j] = NULL;
doc.id[j] = NULL;
doc.ISBN[j] = NULL;
doc.title[j] = NULL;
doc.year[j] = NULL;
}
if (doc.year[i]<1000 || doc.year[i]>9999){
doc.author[i] = NULL;
doc.id[i] = NULL;
doc.ISBN[i] = NULL;
doc.title[i] = NULL;
doc.year[i] = NULL;
}
}
}
break;
}
case 2:
{
int x;
char docid[50];
cout << "IDDOC: ";
cin >> docid[50];
cout << "\n";
for (int i = 0;; i++){
if (docid[50] == doc.id[i]) {
doc.author[i] = NULL;
doc.id[i] = NULL;
doc.ISBN[i] = NULL;
doc.title[i] = NULL;
doc.year[i] = NULL;
x++;
}
}
if (x == 0) cout << "IDDOC khong ton tai." << endl;
break;
}
case 3:
{
int yeardoc[50];
cin >> yeardoc[50];
for (int i = 0;; i++){
if (yeardoc[50] == doc.year[i])
write_output(i, "report.scv");
}
break;
}
case 0:
{
menubegin();
break;
}
menudoc();
}
}
my second function :
int write_output(int i, char* filename){
ios::out;
struct m_doc doc;
// open file for output
ofstream fout(filename);
char * k, *h;
k = strchr(doc.title, ',');
h = strchr(doc.author, ',');
if (k != 0 && h != 0)fout << doc.id[i] << "\n" << doc.ISBN[i] << "\n" << '"' << doc.title[i] << '"' << "\n" << '"' << doc.author[i] << '"' << "\n" << doc.year[i] << endl;
if (k != 0) fout << doc.id[i] << "\n" << doc.ISBN[i] << "\n" << '"' << doc.title[i] << '"' << "\n" << doc.author[i] << "\n" << doc.year[i] << endl;
if (h != 0)fout << doc.id[i] << "\n" << doc.ISBN[i] << "\n" << doc.title[i] << "\n" << '"' << doc.author[i] << '"' << "\n" << doc.year[i] << endl;
// write informations
if (k == 0 && h == 0)fout << doc.id[i] << "\n" << doc.ISBN[i] << "\n" << doc.title[i] << "\n" << doc.author[i] << "\n" << doc.year[i] << endl;
// close file
fout.close();
return 0;
}
anybody im asked said that it was something about scope but none of them know the problem since everything looks fine... please help me , every help is much appreciated :)

Answer 1

I'm taking a guess that you are confused about when to use forward declarations and when to include the structure definition. Here are some guidelines.
Forward Declaration
A forward declaration of a structure is of the form:
struct m_doc doc;
This form is used to resolve definitions, any pointer or reference that refers to the symbol doc. Usually this is placed in a header file before other structure or function declarations to reduce the number of #include files.
Structure Definition
You need to have the structure definition present before any code that:
Creates an instance of the structure,
Or before anything that refers to the details of the structure
(members & methods).
Placing structures in #include files
Rather than typing the structure everywhere it is used, a convenient practice is to place it in a header file, usually ".h" for C language structures and ".hpp" or ".hh" for C++ classes and structures. So when you have a function in a source file that references the structure (or creates an instance of the structure), include the file with the definition before it is used by the function.
Most compilers issue the error
"left of ... must have class/struct/union "
when it can't find the definition or declaration of the structure.