Question

我做了一些实验并收到奇怪的错误消息。在提供的代码中，第9行中有一个错误，它会在模板替换期间带来递归限制，但两个编译器（GCC 4.8.2和CLANG 3.4的日志放在底部）报告

＆＃34;没有名为T＆＃34;的类型。

如果有人能解释编译器的奇怪信息，我将不胜感激。

感谢。

typedef unsigned long size_t;
template<class _T, size_t _N>
struct P;
template<class _T>
struct P<_T, 0> {
    using T= _T;
};
template<class _T, size_t _N>
struct P {
    using T= typename P<_T, 1-_N>::T*; // instead of 1-_N will be _N-1
};
int main(void) {
    using namespace my;
    using namespace std;

    typedef P<int, 0>::T P0; // Ok
    typedef P<int, 1>::T P1; // Ok
    typedef P<int, 2>::T P2; // Error
    typedef P<int, 3>::T P2; // Error
    return 0;
}

-------------- Build: (compiler: GNU GCC Compiler)---------------

g++ -std=c++11 -Wall -fexceptions -Weffc++ -pedantic -std=c++11 -g  -c
: In instantiation of ‘struct P<int, 18446744073709551615ul>’:
:10:42:   required from ‘struct P<int, 2ul>’
:15:26:   required from here
:10:42: error: no type named ‘T’ in ‘struct P<int, 2ul>’
         using T= typename P<_T, 1-_N>::T*; // instead of 1-_N will be _N-1
                                          ^
: In instantiation of ‘struct P<int, 18446744073709551614ul>’:
:10:42:   required from ‘struct P<int, 3ul>’
:16:26:   required from here
:10:42: error: no type named ‘T’ in ‘struct P<int, 3ul>’
Process terminated with status 1 (0 minute(s), 0 second(s))
2 error(s), 6 warning(s) (0 minute(s), 0 second(s))


-------------- Build: (compiler: LLVM Clang Compiler)---------------

clang++ -std=c++11 -Wall -fexceptions  -pedantic -std=c++11 -g -Weffc++     -c
:10:40: error: no type named 'T' in 'P<int, 2>'
        using T= typename P<_T, 1-_N>::T*; // instead of 1-_N will be _N-1
                 ~~~~~~~~~~~~~~~~~~~~~~^
:10:27: note: in instantiation of template class 'P<int, 18446744073709551615>' requested here
        using T= typename P<_T, 1-_N>::T*; // instead of 1-_N will be _N-1
                          ^
:15:17: note: in instantiation of template class 'P<int, 2>' requested here
        typedef P<int, 2>::T P2; // Error
                ^
:10:40: error: no type named 'T' in 'P<int, 3>'
        using T= typename P<_T, 1-_N>::T*; // instead of 1-_N will be _N-1
                 ~~~~~~~~~~~~~~~~~~~~~~^
:10:27: note: in instantiation of template class 'P<int, 18446744073709551614>' requested here
        using T= typename P<_T, 1-_N>::T*; // instead of 1-_N will be _N-1
                          ^
:16:17: note: in instantiation of template class 'P<int, 3>' requested here
        typedef P<int, 3>::T P2; // Error
                ^
2 errors generated.
Process terminated with status 1 (0 minute(s), 0 second(s))
2 error(s), 4 warning(s) (0 minute(s), 0 second(s))

Answer 1

size_t是无符号整数类型，当用2（P<int, 2>::T）实例化模板，并在P<_T, 1-_N>::T*中执行 1-_N 计算时，_std :: numeric_limits :: max（） - 1_这是一个非常大的数字，然后再次实例化P<_T, 1-_N>::T*导致实例化的第一个实例化（ with 2 ），在这种情况下是clang和gcc应该检测到这个特定的实例化正在发生，类型T已经不知道（并且永远不会，因为它将生成实例化的无限循环）。

Answer 2

请注意(1 - (1 - N)) == N。正如编译器所述：

struct P<int, 2ul>取决于struct P<int, 18446744073709551615ul>，它再次依赖于struct P<int, 2ul>（其中T尚未定义（本身有2个指针......）。

在C ++模板部分规范中生成1-N奇怪错误，为什么？

2 个答案: