Non-dependent name lookup and lambda - c++

I have the following code:
template <class T>
class Outer
{
public:
Outer();
template <class U>
void templateFunc()
{
}
private:
class Inner
{
public:
Inner(Outer& outer)
{
outer.templateFunc<int>();
Outer* outer_ptr = &outer;
[outer_ptr]()
{
outer_ptr->templateFunc<int>();
}();
}
};
Inner m_inner;
};
template <class T>
Outer<T>::Outer()
: m_inner(*this)
{
}
int main()
{
Outer<double> outer;
}
As you can see, there is a template class that contains a nested class, which in constructor calls some template method of its enclosing class. AFAIK, even though enclosing class is a template class - for the nested class it is a non-dependent name, so calling its template method without template should not be a problem. The problem happens when I define a lambda inside nested class' constructor, capture pointer to outer class, and try to call the same template method - g++7.2 gives the following compilation error:
prog.cc: In lambda function:
prog.cc:22:41: error: expected primary-expression before 'int'
outer_ptr->templateFunc<int>();
^~~
prog.cc:22:41: error: expected ';' before 'int'
However, g++-5.4 and g++-6.3 compile this code just fine. So it seems that g++-7.2 treats the outer_ptr's type inside the lambda as a dependent name - and I cannot understand why. Can someone explain this to me?

Yes, this is a gcc regression. Filed as 82980.
Here's a reduced example:
template <class T>
struct Outer
{
template <class U>
void f();
void bar(Outer outer) {
[outer](){ outer.f<int>(); };
}
};
int main() { }
outer.f is member access for the current instantiation, so that expression shouldn't count as type dependent, so you shouldn't need to provide the template keyword.

Related

template specialization for member functions in a templated class [duplicate]

I have a templated class with an templated member function
template<class T>
class A {
public:
template<class CT>
CT function();
};
Now I want to specialize the templated member function in 2 ways. First for having the same type as the class:
template<class T>
template<> // Line gcc gives an error for, see below
T A<T>::function<T>() {
return (T)0.0;
}
Second for type bool:
template<class T>
template<>
bool A<T>::function<bool>() {
return false;
}
Here is how I am trying to test it:
int main() {
A<double> a;
bool b = a.function<bool>();
double d = a.function<double>();
}
Now gcc gives me for the line marked above:
error: invalid explicit specialization before ‘>’ token
error: enclosing class templates are not explicitly specialize
So gcc is telling me, that I have to specialize A, if I want to specialize function, right?
I do not want to do that, I want the type of the outer class to be open ...
Is the final answer: it is not possible? Or is there a way?
Yes, this is the problem:
error: enclosing class templates are not explicitly specialized
You cannot specialize a member without also specializing the class.
What you can do is put the code from function in a separate class and specialize that, much like basic_string depends on a separate char_traits class. Then then non-specialized function can call a helper in the traits class.
You can use overload, if you change the implementation.
template <typename T>
class Foo
{
public:
template <typename CT>
CT function() { return helper((CT*)0); }
private:
template <typename CT>
CT helper(CT*);
T helper(T*) { return (T)0.0; }
bool helper(bool*) { return false; }
};
Simple and easy :)

Is this full template specialization, and is it allowed? [duplicate]

I have a templated class with an templated member function
template<class T>
class A {
public:
template<class CT>
CT function();
};
Now I want to specialize the templated member function in 2 ways. First for having the same type as the class:
template<class T>
template<> // Line gcc gives an error for, see below
T A<T>::function<T>() {
return (T)0.0;
}
Second for type bool:
template<class T>
template<>
bool A<T>::function<bool>() {
return false;
}
Here is how I am trying to test it:
int main() {
A<double> a;
bool b = a.function<bool>();
double d = a.function<double>();
}
Now gcc gives me for the line marked above:
error: invalid explicit specialization before ‘>’ token
error: enclosing class templates are not explicitly specialize
So gcc is telling me, that I have to specialize A, if I want to specialize function, right?
I do not want to do that, I want the type of the outer class to be open ...
Is the final answer: it is not possible? Or is there a way?
Yes, this is the problem:
error: enclosing class templates are not explicitly specialized
You cannot specialize a member without also specializing the class.
What you can do is put the code from function in a separate class and specialize that, much like basic_string depends on a separate char_traits class. Then then non-specialized function can call a helper in the traits class.
You can use overload, if you change the implementation.
template <typename T>
class Foo
{
public:
template <typename CT>
CT function() { return helper((CT*)0); }
private:
template <typename CT>
CT helper(CT*);
T helper(T*) { return (T)0.0; }
bool helper(bool*) { return false; }
};
Simple and easy :)

How to derive from a nested class of a variadic template argument?

Given the following two structs, one could derive from both nested 'Nested' classes, and call foo() and bar() from the derived object:
struct WithNested1 {
template<class T> struct Nested {
void foo();
};
};
struct WithNested2 {
template<class T> struct Nested {
void bar();
};
};
struct Test : WithNested1::Nested<Test>,
WithNested2::Nested<Test>
{
};
Test test;
test.foo();
test.bar();
But, if both of the outer classes were passed as variadic template arguments, how would you derive from them?
For example, this fails to compile:
template<typename... Ts>
struct Test : Ts::template Nested<Test>...
{
};
Test<WithNested1, WithNested2> test;
test.foo();
test.bar();
error: 'foo' : is not a member of 'Test'
error: 'bar' : is not a member of 'Test'
strangely, it compiles if the calls to foo() and bar() are removed.
template <typename... Ts>
struct Test : Ts::template Nested<Test<Ts...>>...
{
};
This is the same answer as above but I figured I'd explain how it works. First in your example Test has no template param (which the compiler should warn you of), but which should we give it. The point of CRTP is to give the class you inherit from a template param that is the same as your type, that way it has access to your methods and members through the of the template param. Your type in this case is Test<Ts...> so that is what you have to pass it. As #aschepler already pointed out normally you could use Test by itself but it's not in scope until your already inside the class.
I think this is a cleaner way of doing what you want.
template <typename T>
struct A {
void bar (){
static_cast<T*>(this)->val = 3;
}
};
template <typename T>
struct B {
void foo (){
static_cast<T*>(this)->val = 90;
}
};
template <template<class> class ... Ts>
struct Test : Ts<Test<Ts...>>...
{
int val;
};
int main() {
Test<A,B> test;
test.foo();
test.bar();
return 0;
}
The "injected class name" Test which can be used as an abbreviation of Test<Ts...> is not in scope where you tried to use Nested<Test>, since the class scope does not begin until the { token.
Use
template<typename... Ts>
struct Test : public Ts::template Nested<Test<Ts...>>...
{
};
This works:
template<typename... Ts>
struct Test : Ts::template Nested<Test<Ts...>>...
// ^^^^^^^
{
};
9/2:
[...]. The class-name is also inserted into the scope of the class itself; this is known as the injected-class-name. For purposes of access checking, the injected-class-name is treated as if it were a public member name. [...]
14.6.1/1:
Like normal (non-template) classes, class templates have an injected-class-name (Clause 9). The injectedclass-name can be used as a template-name or a type-name. When it is used with a template-argument-list, as a template-argument for a template template-parameter, or as the final identifier in the elaborated-typespecifier of a friend class template declaration, it refers to the class template itself. Otherwise, it is equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.

template functions in nested template classes

I'm experiencing a problem in some code I've been working on. Here is the most simplified version of it I could create:
template <class A>
class Outer {
public:
template <class B>
class Inner {
public:
template <class C>
void foo(Outer<C>::Inner<C> innerC) { }
};
Inner<A> inner;
};
class X {};
class Y {};
int main() {
Outer<X> outerX;
Outer<Y> outerY;
outerX.foo<Y>(outerY.inner);
}
The error is:
error: expected primary-expression before ">" token
and is triggered at compiletime at the declaration of void foo. What is incorrect about my code that is making this happen?
In words, what I am trying to do is have the nested class be able to take in a nested class with any template type - but of course the nested class's template type depends on the outer class's template type, so I use the :: syntax, but that gives an error.
I understand that what I'm trying to do here might not be a good practice, but the purpose of this question is to understand template syntax better.
There is no conversion from 1 to Inner<C>. Is that an error in your reduced test case, or is it supposed to be:
template <class C>
void foo(C innerC) { }
Update: After the code was fixed, it can be seen that the problem is the lack of template before Inner<C>. Otherwise the compiler will assume that Inner is a value.
template <class C>
void foo(Outer<C>::template Inner<C> innerC) { }

Specialization of templated member function in templated class

I have a templated class with an templated member function
template<class T>
class A {
public:
template<class CT>
CT function();
};
Now I want to specialize the templated member function in 2 ways. First for having the same type as the class:
template<class T>
template<> // Line gcc gives an error for, see below
T A<T>::function<T>() {
return (T)0.0;
}
Second for type bool:
template<class T>
template<>
bool A<T>::function<bool>() {
return false;
}
Here is how I am trying to test it:
int main() {
A<double> a;
bool b = a.function<bool>();
double d = a.function<double>();
}
Now gcc gives me for the line marked above:
error: invalid explicit specialization before ‘>’ token
error: enclosing class templates are not explicitly specialize
So gcc is telling me, that I have to specialize A, if I want to specialize function, right?
I do not want to do that, I want the type of the outer class to be open ...
Is the final answer: it is not possible? Or is there a way?
Yes, this is the problem:
error: enclosing class templates are not explicitly specialized
You cannot specialize a member without also specializing the class.
What you can do is put the code from function in a separate class and specialize that, much like basic_string depends on a separate char_traits class. Then then non-specialized function can call a helper in the traits class.
You can use overload, if you change the implementation.
template <typename T>
class Foo
{
public:
template <typename CT>
CT function() { return helper((CT*)0); }
private:
template <typename CT>
CT helper(CT*);
T helper(T*) { return (T)0.0; }
bool helper(bool*) { return false; }
};
Simple and easy :)