Regarding forward declaration of structs in C++

Question

Hello,

This is my first time working on C++ and I have some problems that needed some help please.

 

I have some sort of circular dependency among my structs. They look like this,

 

struct IF {
  string condition
};

struct Stmt {
	IF ifStat;
};

struct StmtLst {
  vector<Stmt> stmt;
}

struct IF {
	string condition;
	StmtLst thenPart;
  	StmtLst elsePart;
};

 

My IF struct needs to use the StmtLst struct so I had to declare it after the StmtLst struct. But, at the same time the stmt struct would require my IF struct to be declared before it.

 

I tried declared it as

struct IF IF;

to replace the first IF struct because I don't need it. I need the 2nd IF struct, but it's an error saying it uses an undefined struct .

 

Please help me. I'm total clueless what to do now. Thanks!

Answer 1

Mr Bel wrote:

I have some sort of circular dependency among my structs. They look like this,

struct IF {
 string condition
};

struct Stmt {
IF ifStat;
};

struct StmtLst {
 vector<Stmt> stmt;
}

struct IF {
string condition;
StmtLst thenPart;
 StmtLst elsePart;
};

You might be able to get away with this:

struct Stmt;

struct StmtLst {
    vector<Stmt> stmt;
};

struct IF {
    string condition;
    StmtLst thenPart;
    StmtLst elsePart;
};

struct Stmt {
    IF ifStat;
};

Formally, a parameter of std::vector must be a complete type, but I  believe MSVC implementation accepts incomplete types.

If that doesn't work, then you'll have to hold something by pointer, to break the circle. E.g.

struct IF;

struct Stmt {
    IF* ifStat;
};

struct StmtLst {
    vector<Stmt> stmt;
};

struct IF {
    string condition;
    StmtLst thenPart;
    StmtLst elsePart;
};

---

Igor Tandetnik

Answer 2

Mr Bel wrote:

I have some sort of circular dependency among my structs. They look like this,

struct IF {
 string condition
};

struct Stmt {
IF ifStat;
};

struct StmtLst {
 vector<Stmt> stmt;
}

struct IF {
string condition;
StmtLst thenPart;
 StmtLst elsePart;
};

You might be able to get away with this:

struct Stmt;

struct StmtLst {
  vector<Stmt> stmt;
};

struct IF {
  string condition;
  StmtLst thenPart;
  StmtLst elsePart;
};

struct Stmt {
  IF ifStat;
};

Formally, a parameter of std::vector must be a complete type, but I  believe MSVC implementation accepts incomplete types.

If that doesn't work, then you'll have to hold something by pointer, to break the circle. E.g.

struct IF;

struct Stmt {
  IF* ifStat;
};

struct StmtLst {
  vector<Stmt> stmt;
};

struct IF {
  string condition;
  StmtLst thenPart;
  StmtLst elsePart;
};

---

Igor Tandetnik

OMG!!

 

That solved it! I cracked my head for one whole day and still stucked.

 

You're good! Thanks a lot!!!!! Appreciate it very much.