以下代码是 -clause 的用法示例requires：
#include <type_traits>
template <typename T>
requires std::is_integral_v<T>
void take_integral(T value);
它接受一个计算结果为bool值的表达式（std::is_integral_v<T>在本例中），并按预期工作。但是，当使用运算符对此类表达式求反时!，会导致编译错误：
#include <type_traits>
template <typename T>
requires !std::is_integral_v<T>
void take_integral(T value);
来自 GCC 的诊断：
<source>:4:12: error: expression must be enclosed in parentheses
4 | requires !std::is_integral_v<T>
| ^~~~~~~~~~~~~~~~~~~~~~
| ( )
Compiler returned: 1
来自 Clang 的诊断：
<source>:4:12: error: parentheses are required around this expression in a requires clause
requires !std::is_integral_v<T>
^~~~~~~~~~~~~~~~~~~~~~
( )
1 error generated.
Compiler returned: …Clang和GCC(MSVC除外)在std::addressof<int>作为参数传递给模板函数时无法解析模板参数.以下是此类错误的示例:
std::vector<int> v{1,2,3,4,5};
std::vector<int*> pv(iv.size());
std::transform(v.begin(), v.end(), pv.begin(), std::addressof<int>);
铛:
<source>:8:5: error: no matching function for call to 'transform'
std::transform(iv.begin(), iv.end(), piv.begin(), std::addressof<int>);
^~~~~~~~~~~~~~
/opt/compiler-explorer/clang-5.0.0/bin/../include/c++/v1/algorithm:2028:1: note: candidate template ignored: couldn't infer template argument '_UnaryOperation'
transform(_InputIterator __first, _InputIterator __last, _OutputIterator __result, _UnaryOperation __op)
^
GCC:
/opt/compiler-explorer/gcc-7.2.0/include/c++/7.2.0/bits/stl_algo.h:4295:5: note: template argument deduction/substitution failed:
<source>:8:74: note: could not resolve address from overloaded function 'addressof<int>'
std::transform(iv.begin(), iv.end(), piv.begin(), std::addressof<int>);
^
如果参数是a std::addressof,那么该错误将是有意义的,因为UnaryOperator模板参数将是不明确的.但是,编译器不需要演绎什么T是std::addressof<int>,我会在这里不放过任何歧义.
这是我期望的一个有效例子(编译Clang 5和GCC 7.2):
template …我有这个代码:
trick = Just (putStrLn "Hello?")
我想IO ()从Maybe上下文中解开它并调用它.
main = do foo <- trick
foo
但是,这会引发错误:
Couldn't match type ‘IO’ with ‘Maybe’
Expected type: Maybe ()
Actual type: IO ()
我怎么能解决这个问题？
如果数据类型是typedef,如何通过引用传递数组.我正在学习c ++,我阅读了call-by-reference的概念,但是当我按照那个实现时 - 我收到了一个错误(在代码之后粘贴在下面).请问,任何人都可以解释发送数组作为参考调用的最佳方式吗？
#include <iostream>
#include <vector>
using namespace std;
typedef unsigned long ulong;
ulong fib_dynamic(ulong n, ulong &memo[]){
if(n < 2) return 1;
if(memo[n] == 0){
memo[n] = fib_dynamic(n-1, memo) + fib_dynamic(n-2, memo);
}
return memo[n];
}
ulong fib_iterative(ulong n){
ulong fib[n+1];
fib[0] = 1;
fib[1] = 1;
for(int i=2; i<n; i++) {
fib[i] = fib[i-1] + fib[i-2];
}
return fib[n-1];
}
int main(){
ulong n;
cout << "Welcome to Fib Calculator\nEnter the n:";
cin >> n;
ulong …