c++ - 模板 shared_ptr 参数

Question

我收到此代码的编译器错误:

#include <boost/shared_ptr.hpp>
#include <boost/make_shared.hpp>

struct Name
{
};
typedef boost::shared_ptr<Name> NamePtr;

struct Foo
{
    NamePtr name;
};
typedef boost::shared_ptr<Foo> FooPtr;

template<class T>
void setName(T item, NamePtr name = boost::make_shared<Name>() )
{
    item->name = name;  
}

int main()
{
    FooPtr foo = boost::make_shared<Foo>();
    setName(foo);
    return 0;
}

如下:

    main.cpp: error C2780: 'boost::shared_ptr<X> boost::make_shared(A1 &&,A2 &&,A3 &&,A4 &&,A5 &&,A6 &&,A7 &&,A8 &&,A9 &&)' : expects 9 arguments - 0 provided
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(590) : see declaration of 'boost::make_shared'
1>main.cpp: error C2780: 'boost::shared_ptr<X> boost::make_shared(A1 &&,A2 &&,A3 &&,A4 &&,A5 &&,A6 &&,A7 &&,A8 &&)' : expects 8 arguments - 0 provided
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(534) : see declaration of 'boost::make_shared'
1>main.cpp: error C2780: 'boost::shared_ptr<X> boost::make_shared(A1 &&,A2 &&,A3 &&,A4 &&,A5 &&,A6 &&,A7 &&)' : expects 7 arguments - 0 provided
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(480) : see declaration of 'boost::make_shared'
1>main.cpp: error C2780: 'boost::shared_ptr<X> boost::make_shared(A1 &&,A2 &&,A3 &&,A4 &&,A5 &&,A6 &&)' : expects 6 arguments - 0 provided
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(428) : see declaration of 'boost::make_shared'
1>main.cpp: error C2780: 'boost::shared_ptr<X> boost::make_shared(A1 &&,A2 &&,A3 &&,A4 &&,A5 &&)' : expects 5 arguments - 0 provided
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(378) : see declaration of 'boost::make_shared'
1>main.cpp: error C2780: 'boost::shared_ptr<X> boost::make_shared(A1 &&,A2 &&,A3 &&,A4 &&)' : expects 4 arguments - 0 provided
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(330) : see declaration of 'boost::make_shared'
1>main.cpp: error C2780: 'boost::shared_ptr<X> boost::make_shared(A1 &&,A2 &&,A3 &&)' : expects 3 arguments - 0 provided
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(284) : see declaration of 'boost::make_shared'
1>main.cpp: error C2780: 'boost::shared_ptr<X> boost::make_shared(A1 &&,A2 &&)' : expects 2 arguments - 0 provided
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(240) : see declaration of 'boost::make_shared'
1>main.cpp: error C2780: 'boost::shared_ptr<X> boost::make_shared(A1 &&)' : expects 1 arguments - 0 provided
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(198) : see declaration of 'boost::make_shared'
1>main.cpp: error C2783: 'boost::shared_ptr<X> boost::make_shared(void)' : could not deduce template argument for 'T'
1>          c:\users\ebargri\desktop\boost_1_49_0\boost\smart_ptr\make_shared.hpp(121) : see declaration of 'boost::make_shared'

我做错了什么？

如果我用这个替换模板函数，代码可以正常编译:

void setName(FooPtr item, NamePtr name = boost::make_shared<Name>())
{
    item->name = name;  
}

如果我用这个替换它，它也会编译:

template<class T>
void setName(T item)
{
}

如果我按如下方式传入第二个参数，它也会编译:

FooPtr foo = boost::make_shared<Foo>();
NamePtr name = boost::make_shared<Name>();
setName(foo, name);

这也不编译:

setName<FooPtr>(foo);

更新:

这是另一个例子:

#include <boost/shared_ptr.hpp> 
#include <boost/make_shared.hpp> 

template<class T> 
void f(T item, boost::shared_ptr<int> name = boost::make_shared<int>()) 
{ 
} 

int main() 
{ 
    f(0); 
}

Answer 1

这是一个 VS 错误。演示:

namespace foo {
  template <typename T> class A {};
  template<typename T> A<T> mk_A() { return A<T>(); } 
}

template<class T> 
void f(T item, foo::A<int> x = foo::mk_A<int>()) { } // triggers the bug

using foo::mk_A;
template<class T> 
void g(T item, foo::A<int> x = mk_A<int>()) { } // does not trigger the bug


int main () {
  f(0); g(0);
}