Can I forward-declare names -defined in other header in a header files?

Question

I want to create a class Student that has a member of type library std::string but I don't want to include the header <string> in my Student.h and use only forward-declaration:

// Student.h
#ifndef STUDENT_H
#define STUDENT_H

#include <iostream>

typedef class string_ string;

struct Student
{
public:
    Student(const string_&, std::size_t);
    const string_ name()const;
    void setName(const string_&);
    std::size_t age()const;
    void setAge(std::size_t);
private:
    string_* name_ ;
    std::size_t age_;
};

 // Student.cpp
#include "Student.h"
#include <string>

Student::Student(const std::string& str, std::size_t a) :
    name_(&str),
    age_(a)
{}

• When I compile the program I get these errors: ../src/Student.cpp:13:2: error: no declaration matches ‘Student::Student(const string&, std::size_t)’ 13 | Student::Student(const std::string& str, std::size_t a) :

• So can I use forward declaration so that in a header I don't include any header but just forward-declare for the types I need then in source I include the headers?

• I've done this because I'm reading Mastering Qt5 book by Guillaume Lazar in which he gave this example:

  //SysInfoWindowsImpl.h
  #include <QtGlobal>
  #include <QVector>
  #include "SysInfo.h"
  
  
  typedef struct _FILETIME FILETIME;
  
  class SysInfoWindowsImpl : public SysInfo
  {
  public:
      SysInfoWindowsImpl();
      void init() override;
      double cpuLoadAverage() override;
      double memoryUsed() override;
  private:
      QVector<qulonglong> cpuRawData();
      qulonglong convertFileTime(const FILETIME& filetime) const;
  private:
      QVector<qulonglong> mCpuLoadLastValues;
  };
  
  //SysInfoWindowsImpl.cpp
  #include "SysInfoWindowsImpl.h"
  #include <windows.h>
  
  
  SysInfoWindowsImpl::SysInfoWindowsImpl() :
      SysInfo(),
      mCpuLoadLastValues()
  {
  }
  
  void SysInfoWindowsImpl::init()
  {
      mCpuLoadLastValues = cpuRawData();
  }
  
  qulonglong SysInfoWindowsImpl::convertFileTime(const FILETIME& filetime) const
  {
      ULARGE_INTEGER largeInteger;
      largeInteger.LowPart = filetime.dwLowDateTime;
      largeInteger.HighPart = filetime.dwHighDateTime;
      return largeInteger.QuadPart;
  }
  

"The syntax typedef struct _FILETIME FILETIME is a kind of forward declaration for FILENAME syntax. As we only use a reference, we can avoid including the tag in our file SysInfoWindowsImpl.h and keep it in the CPP file." from the book.

• So can someone explain to me how could he use typedef struct _FILETIME which is defined in windows.h? Thank you.

Answer 1

No, you cannot today (in may 2020). See also this and n3337 (the C++11 standard).

But C++20 or later might add modules, and then things become different.

Practically speaking, for a small program (e.g. less than a few dozen thousands lines of your C++ code) a common approach is to have a single header file -which you would #include in each of your translation units, and that common master header file would #include many standard headers. With recent GCC, you could use precompiled headers (and practically speaking, that works the best with a single master header, perhaps #include -ing sub header files; see this explanation)

Consider also using a good enough build automation toolset. On Linux, that might be a combination of GCC or Clang, ccache, with make or ninja, etc...

Answer 2

Yes, but only if they match.

You forward declared a global type string, not in namespace std.

You could probably make it work with namespace std {} but then your program would have undefined behaviour, because you're not allowed to declare new things in that namespace (with a few exceptions).

In general, you want to avoid forward declarations for anything but your own classes.

Just #include <string>. If doing so is causing problems, you should resolve those independently.