我正在开发一个项目,似乎 clang 无法生成有效的字节码(由于链接器无法链接,因此在链接时找不到模板类中的某些 static constexpr
)
我可以使用类中的静态 getter 来修复它,但这会导致一些非常丑陋/重载的代码。
这是一个“最小”的代码示例,它使错误(这是一个错误吗?)出现。 不幸的是,在这里,g++ 将产生相同的链接错误。
我问这是否是编译器错误,或者我只是做错了什么,以及是否有解决方案可以避免此错误。 (如果我做错了什么,为什么相同的构造在我更大的项目中起作用??)
注意:更大的项目在 github 上名为 yaggler
,但仍处于其生命的早期阶段
#include <type_traits>
#include <iostream>
// exemple vector classes
struct vector2
{
constexpr vector2(unsigned int _x = 0, unsigned int _y = 0) : x(_x), y(_y) {} // for clang
unsigned int x;
unsigned int y;
};
struct vector3 : public vector2 // uh ;)
{
constexpr vector3(unsigned int _x = 0, unsigned int _y = 0, unsigned int _z = 0) : vector2(_x, _y), z(_z) {} // for clang
unsigned int z;
};
// simple templated generic vector type
// we could make a more generic one, but this would require something like a tuple.
template<unsigned int... Vals>
struct vector
{
static_assert(!(sizeof...(Vals) + 1), "[...]");
};
template<unsigned int X>
struct vector<X>
{
using vec_type = unsigned int;
static constexpr unsigned int value = X;
};
template<unsigned int X, unsigned int Y>
struct vector<X, Y>
{
using vec_type = vector2;
static constexpr vector2 value = vector2(X, Y);
};
template<unsigned int X, unsigned int Y, unsigned int Z>
struct vector<X, Y, Z>
{
using vec_type = vector3;
static constexpr vector3 value = vector3(X, Y, Z);
};
// a simple wrapper
template<typename V>
struct some_wrapper
{
static constexpr typename V::vec_type value = V::value;
};
// a dummy function that print something to stdout.
void do_something(int32_t id, const vector3 &value)
{
std::cout << id << " " << value.z << std::endl;
}
void do_something(int32_t id, const vector2 &value)
{
std::cout << id << " " << value.y << std::endl;
}
void do_something(int32_t id, int value)
{
std::cout << id << " " << value << std::endl;
}
// the class used to create the error
template< typename... Args>
class exemple
{
private:
// an initialisation that recurse over the Args... template arguments
template<typename Current, typename... Others>
void __rec_init() const
{
do_something(0, Current::value);
__rec_init<Others...>();
}
// end of recursion
template<size_t = 0>
void __rec_init() const {}
// launch the recursion
void tpl_init() const
{
__rec_init<Args...>();
}
public:
exemple()
{
tpl_init();
}
};
int main()
{
// and here, we get a linker error.
exemple<some_wrapper<vector<4, 4, 5>>, some_wrapper<vector<4, 1>>, some_wrapper<vector<9>>>();
}
编辑:仅提及 gcc 和 clang 版本:gcc 4.7.3/4.8.2 和 clang 3.2/3.3
最佳答案
vector
的特化2 个和 3 个模板参数的类模板有 static constexpr
数据成员value
没有命名空间范围定义的文字类型(分别为 vector2
和 vector3
)。
您需要它们,因为您使用 value
当它绑定(bind)到传递给do_something
时的引用参数时功能。
§9.4.2/3 [class.static.mfct]
If a non-volatile
const static
data member is of integral or enumeration type, its declaration in the class definition can specify abrace-or-equal-initializer
in which everyinitializer-clause
that is anassignment- expression
is a constant expression (5.19). Astatic
data member of literal type can be declared in the class definition with theconstexpr
specifier; if so, its declaration shall specify abrace-or-equal-initializer
in which everyinitializer-clause
that is anassignment-expression
is a constant expression. [Note: In both these cases, the member may appear in constant expressions. —end note ] The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer.
编辑:纠正我自己,实际上是 some_wrapper<T>::value
需要这个定义(尽管如此,出于上述原因)。所以你需要的是在some_wrapper
定义之后的命名空间范围内的这个。 :
template<typename V>
constexpr typename V::vec_type some_wrapper<V>::value;
之后,您的代码 compiles and runs .
关于c++ - 使用 clang 和 g++ 编译此 "simple"程序时出现链接错误,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/21344031/