How do I inherit from a virtual template class, in this code:
// test.h
class Base {
public:
virtual std::string Foo() = 0;
virtual std::string Bar() = 0;
};
template <typename T>
class Derived : public Base {
public:
Derived(const T& data) : data_(data) { }
virtual std::string Foo();
virtual std::string Bar();
T data() {
return data_;
}
private:
T data_;
};
typedef Derived<std::string> DStr;
typedef Derived<int> DInt;
// test.cpp
template<typename T>
std::string Derived<T>::Foo() { ... }
template<typename T>
std::string Derived<T>::Bar() { ... }
When I try to use the DStr or DInt, the linker complain that there are unresolved externals, which are Derived<std::string>::Foo() and Derived<std::string>::Bar(), and the same for Derived<int>.
Did I miss something in my code?
EDIT:
Thanks all. It's pretty clear now.
You need to define template<typename T> std::string Derived<T>::Foo() { ... } and template<typename T>
std::string Derived<T>::Bar() { ... } in the header file. When the compiler is compiling test.cpp it doesn't know all the possible values of T that you might use in other parts of the program.
I think there are some compilers that have connections between the compiling and linking steps that notice references to missing template instantiations and go instantiate them from the .cpp file where they are declared. But I don't know which they are, and the functionality is exceedingly rare to find.
If you define them in the header file most compilers will emit them as a 'weak' symbol into every compilation unit in which they're referenced. And the linker will toss out all except for one definition of the weak symbol. This causes extra compilation time.
Alternately, there are syntaxes for explicitly instantiating templates and forcing the compiler to emit definitions right there. But that requires you to be aware of all the values T could possibly have and you will inevitably miss some.
You have to ensure that the member functions are instantiate for all the required types somewhere.
Usually this is accomplished by defining template functions inline in the header file where they are declared so that any use of the functions will cause them to be instantiated.
As an alternative, you can use explicit instantiations in the source file where you define them but this does require you to know in advance all the types that your template will ever be instantiated for.
This doesn't really have much to do with derivation. It's just a general rule with templates: with most compilers (anything but Comeau) you have to put the full implementation of a template where it's visible in every translation unit that will instantiate that template -- normally in the header.
Even with Comeau, you have to use the export keyword to make things work right. Since They're the only ones that implement export, chances are pretty good that you don't care much though.
Related
I am having a problem with the changes that were made to the way C++ templates are compiled, between the C++17 and 19 standards. Code that used to compile in VS2017 throws a compiler error since I upgraded to VS2019 or VS2022.
Situations have to do with the fact that the compiler now runs a basic syntax check on the template definition when it sees this definition ("first pass") and not only when the template is actually used.
Code example 1:
class Finder
{
template<typename T>
T convert_to(HANDLE h)
{
return Converters::Converter<T>::Convert(get_data(h));
}
};
Here, the template class Converter<> resides in namespace Converters, and get_data is a member function of Finder which returns something that can be passed into the Convert function.
Since we're dealing with templates, this code sits in a header file "Finder.h". The header file doesn't #include "Converters.h". Finder.h is shared across several projects, some of which don't even know the Converters.h file namespace.
As long as no code calls the MyClass::convert_to<> function, this compiles in VS2017, but not so in VS2019 and VS2022:
error C3861: 'Converters': identifier not found
The obvious solution is, of course, to #include "Converters.h" either in this header file, or in the precompiled headers file. However, as was said, Converters.h is not known in all places which use MyClass. Another solution would be to use archaic #define CONVERTERS_H in the Converters.h header and enclose the function definition in #ifdef CONVERTERS_H, but this looks really ugly.
My question is: Is there a way to prevent the compiler from doing this "first pass"? Or to re-write this code so that it compiles? I don't mind if it's MS specific; no other compiler will ever see the code.
Code example 2:
class MyClass2
{
template<class T>
static void DoSomething(T* ptr) { static_assert(false, "Don't do this"); }
// lots more member functions, most of them 'static'
};
template<> void MyClass::DoSomething(CWnd* ptr) { /*some useful code*/ }
/// and some more specializations of DoSomething
The intention is that the static_assert should emit an error message whenever DoSomething is called with an argument for which no explicit specialization of this template function is defined. This worked in VS2017, but in VS2022, the "first pass" of the compiler triggers the static_assert.
Again, I wonder how I could achieve this effect, other than by replacing the static_assert by a run-time assertion.
Or am I thinking into a completely wrong direction?
Thanks
Hans
The first case requires a forward declaration of some kind, that's unavoidable.
The second case, though, can be handled with just a minor change.
#include <type_traits>
class CWnd {};
class MyClass2
{
public:
template<class T, class Y=T>
static void DoSomething(T* ptr) { static_assert(!std::is_same_v<Y,T>, "Don't do this"); }
};
template<> void MyClass2::DoSomething(CWnd* ptr) { /*some useful code*/ }
void foo()
{
int a;
CWnd b;
MyClass2::DoSomething(&a); // ERROR
MyClass2::DoSomething(&b); // OK
}
(partial answer)
To fix MyClass2, the usual trick is to make false depend on T, so that the first pass does not trigger the assert.
// dependent false
template <typename>
constexpr bool dep_false() { return false; }
class MyClass2
{
template<class T>
static void DoSomething(T* ptr) {
static_assert(dep_false<T>(), "Don't do this");
}
// lots more member functions, most of them 'static'
};
// specialization example
template<>
void MyClass2::DoSomething<int>(int* ptr) {
std::cout << "int* is OK\n";
}
I have a set of templates where each base template has some large specializations and many of the other methods are mainly casts around types and constructors and operator=(). I made templates to factor out all those little methods as inlines and put the fat ones in the CPP files for a shared library. In windows it compiles and links fine into a DLL. It all does what I want on MSC but on GCC with the specialization code in the C++ file many of the specializations occur "after instantiation" with GCC.
Structurally the code is like the below. (the real case is more messy) I'm not sure what I need to do as the template method declarations that call their superclass methods (from other templates) are where the method code is "instantiated" for their superclass templates. What I need to do is somehow defer the instantiation until after the header declarations to happen in the .cpp file where the specialization's method bodies for the library are compiled.
// header file .h
template<class SubClassT, class CharT>
class XBaseT
: public YBaseT<char_object<CharT>>
{
typedef XBaseT<XObject,CharT> super;
public:
typedef char_object<CharT> ObjT; // seen by subclasses
inline XBaseT() {}
inline XBaseT(ObjT &p); // note no body, then intention is to have this specialization in library, not header.
...
};
template<>
class XClass<XObject>
: public XBaseT<XObject,char>
{
typedef XBaseT<XObject,char> super;
public:
inline XClass() {}
inline XClass(ObjT &p) : super(p) {}
...
};
template<>
class XObject
: public ContainerT<XObject>
{
typedef ContainerT<XObject> super;
public:
inline XObject() {}
static ContainerT<XObject> *static create(InitStruct &is);
...
};
// specializations in cpp file
tenplate<>
ContainerT<XObject> *
create(InitStruct &is) {
// big code here needs to be in CPP file for library
}
// specialization for <XObject,char> happens "after instantiation" even thoght the method in it's template class is declared with no body.
template<>
XBaseT<XObject, char>::XBaseT(ObjT *p) {
p_ = XObject::create(p);
}
The specialization just above in the.cpp file barfs on GCC as it is "after instantiation",
The instantiation in GCC happens when the "inline XClass(ObjT *p) : super(p) {}" constructor in XClass is declared. And that is so tiny I would like it to be inline as it should optimize out to nothing. On MSC the code is not instantiated until explicitly done so in the .cpp file. and the with MSC the linker seems to sort out what is needed and the compiler warns of missing methods declared but NOT instantiated there. In a declaration of
template class XBaseT;
template class ContainerT;
template class XClass;
The issues is some of the methods specialized for these templates are dependent on more than one of the templates and the specializations need to happen after the declarations.
How do I using "typename" or other template features get this for compile and link under CCC as opposed to windows MSC ?
I want to use the pimpl idiom to avoid having users of my library need our external dependencies (like boost, etc) however when my class is templated that seems to be impossible because the methods must be in the header. Is there something I can do instead?
If the class is templated, your users essentially need to compile it (and this is literally true in the most widely-used C++ implementations) and so they need your external dependencies.
The simplest solution is to put the bulk of your class's implementation in a non-template base class (or encapsulated member object of some class). Solve the module-hiding problem there.
And then write the template derived (or enclosing) class to add type safety to it.
For example, suppose you have a template that provides the amazing ability to allocate on first access (omitting the necessary copy constructor, assignment, destructor):
template <class T>
class MyContainer
{
T *instance_;
public:
MyContainer() : instance_(0) {}
T &access()
{
if (instance_ == 0)
instance_ = new T();
return *instance_;
}
};
If you wanted the "logic" to be separated into a non-template base class, you'd have to parameterise the behaviour in the non-template way, which is to say, use virtual functions:
class MyBase
{
void *instance_;
virtual void *allocate() = 0;
public:
MyBase() : instance_(0) {}
void *access()
{
if (instance_ == 0)
instance_ = allocate();
return instance_;
}
};
Then you can add the type-awareness in the outer layer:
template <class T>
class MyContainer : MyBase
{
virtual void *allocate()
{ return new T(); }
public:
T &access()
{ return *(reinterpret_cast<T *>(MyBase::access())); }
};
i.e. You use virtual functions to allow the template to "fill in" the type-dependent operations. Obviously this pattern would only really make sense if you have some business logic that is worth the effort of hiding.
You can explicitly instantiate templates in the source file, but that is possible only if you know what the template type is going to be. Otherwise, do not use pimpl idiom for templates.
Something like this :
header.hpp :
#ifndef HEADER_HPP
#define HEADER_HPP
template< typename T >
class A
{
// constructor+methods + pimpl
};
#endif
source.cpp :
#include "header.hpp"
// implementation
// explicitly instantiate for types that will be used
template class A< int >;
template class A< float >;
// etc...
There are two general solutions:
while the interface depends on some type T, it defers to a more weakly typed implementation (e.g. one using void* pointers directly or trough type erasure), or
you support only a specific and quite limited number of types.
The second solution is relevant for e.g. char/wchar_t-dependent stuff.
The first solution was quite common in the early days of C++ templates, because at that time compilers were not good at recognizing commonalities in the generated machine code, and would introduce so called “code bloat”. Today, much to the surprise of any novice who tries it out, a templated solution can often have smaller machine code footprint than a solution relying on runtime polymorphism. Of course, YMMV.
Cheers & hth.,
What is the best pratice in regards to defining a template in C++?
template <class T>
class A
{
private:
// stuff
public:
T DoMagic()
{
//method body
}
}
Or:
template <class T>
class A
{
private:
// stuff
public:
T DoMagic();
}
template <class T>
A::T DoMagic()
{
// magic
}
Another way?
I seem to stumble over some controversy regarding this subject.
So; What path to choose?
This is completely a matter of style. That said however:
choose a way and stick to it -- either all inline, or all out, or mixed based on some rule
personally I use a 3 line rule. If the method body in the template is longer than 3 lines I move it outside.
There's no real reason not to include all definitions inline (they are inline from the compilers POV anyway), however, many people argue that keeping them separate is more clean, and allows the class definition to be more readable.
Use an increasing level of separation as the templates you write grow larger and more complex.
Performance will be the same no matter how you separate the definitions from the declarations, so your main concern here should be readability and maintainability.
When writing a simple template used only in one place, declare and define it inline with the declarations right in the CPP file where you're going to use it. There's no reason to force a global recompile if only one block of code needs this template.
file.cpp
template<class Gizmo> bool DoSomethingFancy()
{
// ...
}
For small template utilities used across translation units, define and declare them together in an H file:
utility.h
template<class Gizmo> bool DoSomethingUseful()
{
// ...
}
As your templates become more complex it will become more important to be able to view the declaration separately from the definition. At first, keep everything separate but in the same file:
utility.h
template<class Type> class Useful
{
bool FunctionA();
bool FunctionB();
};
template<class Type> bool Useful<Type>::FunctionA()
{
// ...
}
template<class Type> bool Useful<Type>::FunctionB()
{
// ...
}
But eventually even this will become unwieldly. When it does, separate it in to a header file for the declarations, and an INC file for the definitions. At the end of the header file, #include the INC file:
utility.h :
template<class Type> class MoreUseful
{
bool FunctionA();
bool FunctionB();
};
#include "utility.inc"
utility.inc :
template<class Type> bool MoreUseful<Type>::FunctionA()
{
// ...
}
template<class Type> bool MoreUseful<Type>::FunctionB()
{
// ...
}
This is a religious (style) issue. I prefer to define my functions outside of the template declaration for classes that have more than one method or the few methods are simple.
In either case, my understanding is that the template declaration and the method definitions must be in the same translation unit. This is because the template is more like a stencil, the compiler plugs a given type into the stencil and generates code for the given type.
Whatever you decide, just be consistent.
I usually define all the methods outside but each time I wish C++ had some sort of "template blocks":
template <class T>
struct A
{
T foo();
T bar(T * t);
T baz(T const & t);
};
template <class T> // Made-up syntax
{
T A::foo()
{
//...
}
T A::bar(T * t)
{
//...
}
T A::baz(T const & t)
{
//...
}
}
If the functions are non-trivial (i.e. more than one or two lines), consider defining them separately. This makes the interface of the class much easier to navigate, read and understand for the users of your class, who most likely shouldn't have to look at the actual implementation of each method.
For a one-off instance like your example, it makes little difference.
What happens when there are lots of templates with lots of variations? It then helps to put similar types of apples together, and similar types of oranges together away from them. Of course, this must all be done as intuitively as practical. That is greatly affected by the culture of programmers working with the code.
This code:
template <typename T>
struct A
{
T t;
void DoSomething()
{
t.SomeFunction();
}
};
struct B
{
};
A<B> a;
is easily compiled without any complaints, as long as I never call a.DoSomething().
However, if I define DoSomething as a virtual function, I will get a compile error saying that B doesn't declare SomeFunction. I can somewhat see why it happens (DoSomething should now have an entry in the vtable), but I can't help feeling that it's not really obligated. Plus it sucks.
Is there any way to overcome this?
EDIT 2: Okay. I hope this time it makes sence:
Let's say I am doing intrusive ref count, so all entities must inherit from base class Object. How can I suuport primitive types too? I can define:
template <typename T>
class Primitive : public Object
{
T value;
public:
Primitive(const T &value=T());
operator T() const;
Primitive<T> &operator =(const T &value);
Primitive<T> &operator +=(const T &value);
Primitive<T> &operator %=(const T &value);
// And so on...
};
so I can use Primitive<int>, Primitive<char>...
But how about Primitive<float>? It seems like a problem, because floats don't have a %= operator. But actually, it isn't, since I'll never call operator %= on Primitive<float>.
That's one of the deliberate features of templates.
If, for some reason, I would define operator %= as virtual. Or, if i'll pre-export Primitive<float> from a dll to avoid link errors, the compiler will complain even if I never call operator %= on a Primitive<float>. If it would just have fill in a dummy value for operator %= in Primitive<float>'s vtable (that raises an exception?), everything would have been fine.
Put the virtuals into selectable base classes...
struct Jumper
{
virtual void Jump =0;
};
struct Crawler
{
virtual void Crawl() =0;
};
struct JumperCrawler:
public Jumper,
public Crawler
{
};
template<typename T, typename Methods>
class ICanBoostJumpingAndCrawling :
public Methods
{
T t;
};
Now you can use ICanBoostJumpingAndCrawling with Jumper,Crawler or JumperCrawler supplied as the Methods template parameter; realizing that you need to be derived from it so that you can implement Jumping and or Crawling in a subclass.
FYI, This makes the name "ICanBoostJumpingAndCrawling" completely misleading because it may or may not be able to do that; which means it should be renamed to something like "Booster".
That's not a bug, it's a feature -- seriously. At one time, most compilers would NOT compile the code, for exactly the reason you give. They've since been updated to compile it, partly because the standard requires it.
There was feature called "Concepts" in the C++ 0x standard for quite a while that would have allowed you to specify that T needed a member named 'SomeFunction', including its return type, argument type(s), etc.
Sadly, at the last meeting of the standard committee, they decided finishing up Concepts would delay the standard longer that most people wanted to wait, so they removed them.
Though it's not nearly as good, Boost does have a Concept Checking library that can do what you're asking about.
One way to overcome this is to specialize A for template argument B and not declare DoSomething()
template <>
struct A<struct B>
{
T t;
};
Ofcourse this means that you now have to implement the entire A struct from scratch.
So the compiler should be able to work out within the compilation unit just what is in use. As soon as you start to involve multiple compilation units it no longer has the limited scope and takes the necessary step to ensure that all of the class can compile.
For exporting from a library don't force the pre export, you can ignore the warnings about not exporting the templates as long as you compile all the code with the same compiler the template will be compiled the same in all locations, only compile what is necessary within each compilation unit.
To work around the problem with virtual, then the best you can do is to defer the issue to some other class - don't put virtual in the template.
Perhaps
addition of a 'traits' portion to your template would allow a cheap way out.
use multiple inheritance to define the composite template, like using shims
ie.
template <typename T>
class Additive
{
public:
Primitive<T> &operator =(const T &value);
Primitive<T> &operator +=(const T &value);
};
template <typename T>
class Multiplicative
{
public:
Primitive<T> &operator *=(const T &value);
Primitive<T> &operator /=(const T &value);
};
template <typename T>
class Integers : public Additive<T>, public Multiplicative<T>;
I'd really go back and ask if your abstracting the right level of information for making the template.