模板構造函數失去右值

Question

我使用的是模板的構造在我班的一個

有誰告訴我爲什麼這個代碼，沒有警告或錯誤以及代碼（創建默認FOO右值測試），但一個行編譯剛消失了！

#include <iostream> 

class Foo 
{ 
    int _value; 
public: 
    Foo() 
    :_value(0) 
    { 
     std::cout << __PRETTY_FUNCTION__ << "value[" << _value << "]" << std::endl; 
    } 
    Foo(int value) 
    :_value(value) 
    { 
     std::cout << __PRETTY_FUNCTION__ << "value[" << _value << "]" << std::endl; 
    } 
    Foo(Foo&& foo) = delete; 
    Foo(const Foo& foo) = delete; 
    friend std::ostream& operator << (std::ostream& output, const Foo& foo) 
    { 
     output << foo._value; 
     return output; 
    } 
}; 
class Test 
{ 
public: 
    template <typename Type> 
    Test(Type&& value) 
    { 
     std::cout << __PRETTY_FUNCTION__ << " with value[" << value << "]" << std::endl; 
    } 
    template <typename Type> 
    void fn(Type&& value) 
    { 
     std::cout << __PRETTY_FUNCTION__ << " with value[" << value << "]" << std::endl; 
    } 
}; 

int main() 
{ 
    std::cout << "//----- test fn with foo rvalue ---------------" << std::endl; 
    Test test3(3); 
    test3.fn(Foo()); 
    std::cout << "//----- test with int rvalue ------------------" << std::endl; 
    Test test4(1+3); 
    std::cout << "//----- create test with default foo rvalue ---" << std::endl; 
    Test test5(Foo()); 
    std::cout << "//----- create test with foo rvalue -----------" << std::endl; 
    Test test7 (Foo(1+6)); 
    std::cout << "//----- create test with moved foo rvalue -----" << std::endl; 
    Test test8(std::move(Foo())); 

    return 0; 
} 

這將產生以下結果

//----- test fn with foo rvalue --------------- 
Test::Test(Type&&) [with Type = int] with value[3] 
Foo::Foo()value[0] 
void Test::fn(Type&&) [with Type = Foo] with value[0] 
//----- test with int rvalue ------------------ 
Test::Test(Type&&) [with Type = int] with value[4] 
//----- create test with default foo rvalue --- 
//----- create test with foo rvalue ----------- 
Foo::Foo(int)value[7] 
Test::Test(Type&&) [with Type = Foo] with value[7] 
//----- create test with moved foo rvalue ----- 
Foo::Foo()value[0] 
Test::Test(Type&&) [with Type = Foo] with value[0] 

我使用克++（Ubuntu的4.8.2-19ubuntu1）4.8.2與STD = C++ 1Y

如果有幫助，我添加了一行在端

std::cout << test5; 

而編譯器產生一個警告

warning: the address of ‘Test test5(Foo (*)())’ will always evaluate as ‘true’ [-Waddress] 

Answer 1

Test test5(Foo()); 

它是函數聲明，而不是對象的創建。對於創建對象，你可以使用例如以下

Test test5((Foo())); 
Test test5 = Test(Foo()); 
Test test5{Foo()};