stdfwd

Forward declarations for C++ standard library.

This project was inspired Philip-Trettner/cpp-std-fwd.

Project cpp-std-fwd is as a proof-of-concept and last comment was Aug 25, 2019 so this project implemented all forward declarations for C++99/C++03/C++11/C++14/C++17

Table of Contents

• Usage instead standard library
• Easy way to find class from standard library
• Related repository
• Examples of usage
• FAQ
• TODO

Usage instead standard library

Just add stdfwd for standard header:

• Before:

#include <string>

• After:

#include <stdfwd/string>

For example:

• Old code:

#include <string>

class Person
{
public:
    virtual std::string getName() const = 0;
};

• New code:

#include <stdfwd/string>

class Person
{
public:
    virtual std::string getName() const = 0;
};

For some classes need change namespace std to stdfwd because they have default template arguments, for example vector:

• Old code:

#include <vector>

class Conference
{
public:
    virtual std::vector<Person> getParticipants() const = 0;
};

• New code:

#include <stdfwd/vector>

class Conference
{
public:
    virtual stdfwd::vector<Person> getParticipants() const = 0;
};

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Easy way to find class from standard library

Standard headers have several classes so if you need only one class you include all classes from the header. Since v0.3 version standard headers was splited to headers that have only one class and name of headers is equal class name, for example:

• Old code:

#include <memory>

class Object
{
public:
    virtual std::shared_ptr<int> getSharedPtr() const = 0;
};

• New code:

#include <stdfwd/shared_ptr>

class Object
{
public:
    virtual std::shared_ptr<int> getSharedPtr() const = 0;
};

In this example you include forward declaration only for std::shared_ptr

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Examples of usage

• cppinclude

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Related repository

• Philip-Trettner/cpp-std-fwd

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FAQ

• Why?

Each std header increases compiling time in case where forward declaration will be enough

• Why the namespace stdfwd?

Some classes like std::vector have default template arguments which must only appear on a declaration once. If they appear on the definition then they cannot appear on the forward declaration. Thus, all forward declarations in std are without default arguments and stdfwd adds typedefs with the appropriate defaults.

• Won't this be obsolete with modules?

Yes and no. In C++20, std itself is not yet modularized. Additionally, it is not clear if all code bases can be immediately migrated to modules. This std forward header is a small and easy-to-implement non-intrusive change with big payoffs for many code bases.

• Isn't adding declarations to std undefined behavior?

Yes and no. We have header iosfwd that contains forward declarations for the Input/output library and some compilers have own similar headers with forward declarations. Plus the standard is expected to grow and if next C++ release will have forward declarations this project can be useful for compilers that doesn't support new standard

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TODO

• add forward declarations for C++20

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