我在模板的帮助下以特定的方式调用API，我在传递常量参数时留下了一个问题。
我尝试 int bound ：
template <typename F, typename ...Ts>
static int f3(int bound, CString file, int line, CString Caller,
CString f_name, F f, Ts&& ...ts) {
int err = fn(bound, file, line, Caller, f_name,
f, std::tuple<Ts...>(ts...), seq3<bound>{}, // error C2975
seq1<sizeof...(Ts)>{});
return err;
}
主要：
int const bound;
bound = 4;
err = fn(bound, api(GetModuleFileName), rval, nullptr, path, MAX_PATH, L"EXE-path");
compiler error C2975: 'N': invalid template argument for 'seq3', expected compile-time constant expression
如何解决这个问题？
我现在的解决方法：
err = f3（api（GetModuleFileName），rval，nullptr，path，MAX_PATH，L“EXE-path”）;
f3是具有3个参数的API的特化，因为我现在无法传递上限 - 在本例中为4 - 用于生成序列：＆lt; 1,2,3＆gt;。 调用具有3个参数的API需要此序列，其中tupel从f3（）中的参数rval开始。
背景：
api是一个#define
f3调用API。
f3处理序列/ tupel的0位置的API的返回值。
f3使用所有参数调用另一个用于记录调试信息的可变参数函数。
两个函数调用的一个tupel和两个序列。
问题：
我想传递一个参数来控制一个序列的上限，而不是由tupel-size给出，而是由API函数签名。
对于所有API，我只需要一个fn（）而不是f0（），f1（），f2（），f3（）.....对于具有0,1,2,3 ...参数的API。
我想要这样的事情：
err = fn（seq3＆lt; 4＆gt;，api（GetModuleFileName），rval，nullptr，path，MAX_PATH，L“EXE-path”）
这是我的工作代码：
#include <windows.h>
#include <atlstr.h>
#include <tuple>
#include <utility>
template <int ... Ns> struct seq_3 {};
template <int ... Ns> struct seq3_n {};
template <int I, int ... Ns> struct seq3_n<I, Ns...>{
using type = typename seq3_n<I - 1, I - 1, Ns...>::type;};
template <int ... Ns> struct seq3_n<1, Ns...>{
// skip first argument : rval, because it doesn't fit to API,
// but needed for calling other function
using type = seq_3<Ns...>; };
template <int N>
using seq3 = typename seq3_n<N>::type;
template <int ... Ms> struct seq_1 {};
template <int ... Ms> struct seq1_n {};
template <int J, int ... Ms> struct seq1_n<J, Ms...>{
using type = typename seq1_n<J - 1, J - 1, Ms...>::type; };
template <int ... Ms> struct seq1_n<0, Ms...> {
using type = seq_1<Ms...>; };
template <int M>
using seq1 = typename seq1_n<M>::type;
template <typename F, typename TUP, int ... INDICES3, int ... INDICES1>
static int fn(CString file, int line, CString Caller, CString f_name,
F f, TUP tup, seq_3<INDICES3...>, seq_1<INDICES1...>) {
int err = 0;
// handling of rval = first element of tuple
std::get<0>(tup) = f(std::get<INDICES3>(tup) ...); // calling API
err = GetLastError();
/* calling next function (variadic too) with same tupel, but other sequence
myOpenDebugOutputString(project, file, line, Caller, f_name, std::get<INDICES1>(tup) ..., "stop");
*/
return err; }
template <typename F, typename ...Ts>
static int f3(CString file, int line, CString Caller, CString f_name,
F f, Ts&& ...ts) {
int err = fn(file, line, Caller, f_name,
f, std::tuple<Ts...>(ts...), seq3<4>{}, // sequence fixed by f3
seq1<sizeof...(Ts)>{}); // 3 arguments api + skip 1 rval = 4
return err; // given by signature of API
}
int main() {
// for calling simple API GetModulFileName with 3 arguments
// returns len(path)
wchar_t path[MAX_PATH];
DWORD rval = 0;
int err = 0;
rval = GetModuleFileName( nullptr, path, MAX_PATH);
err = GetLastError();
#define api(a) __FILE__, __LINE__, __func__, L#a, a
// L#a becomes L"GetModuleFileName"
err = f3(api(GetModuleFileName), rval, nullptr, path, MAX_PATH, L"EXE-path");
return 0; }
提前致谢。
P.S。 我正在使用Microsoft Visual Studio 2015
我尝试在Richard Hodges解决方案的模板api_call中关注。
std::tuple<GivenArgs...> tup(args...);
// OK, but only for an api with 3 arguments
callsite.function(std::get<0>(tup), std::get<1>(tup), std::get<2>(tup));
// compiler error too many arguments
callsite.function(std::forward<GivenArgs>(args)..., seq1<callsite.nofArgs()>{});
// compiler error too few arguments
callsite.function(tup, seq1<callsite.nofArgs()>{});
备注：
SEQ1＆LT 3的密度; = seq_1＆lt; 0,1,2＆gt;
callsite.nofArg（）= 3
如何获得正确数量的参数？
答案 0 :(得分：1)
我不清楚你想如何处理错误等等。我假设返回一个错误代码和值的元组。
这是一个我认为会做你想要的一般模式。您需要注意emit_log的特化和重载，特别是对于可能不是空终止或包含非打印字符的字节数组。
为了方便起见，我使用了窄字符，但是这个想法适用于带有一些编辑的宽字符。
注意：在linux gcc上编辑，所以我模拟了windows API。
#include <cstdint>
#include <utility>
#include <iostream>
#include <variant>
#define WINAPI
#define _In_opt_
#define _Out_
#define _In_
struct _hmodule {};
using HMODULE = _hmodule*;
using LPTSTR = char*;
using LPCTSTR = const char*;
using DWORD = std::uint32_t;
extern DWORD WINAPI GetModuleFileName(
_In_opt_ HMODULE hModule,
_Out_ LPTSTR lpFilename,
_In_ DWORD nSize
);
extern WINAPI DWORD GetLastError();
template<class Ret, class...Args>
struct api_call_site
{
const char* file;
int line;
const char* current_function;
const char* called_function;
Ret (* function)(Args...);
};
template<class Ret, class...Args>
auto make_api_call_site(const char* file, int line, const char* callername, const char* calleename, Ret (* WINAPI callee)(Args...))
{
return api_call_site<Ret, Args...>
{
file,
line,
callername,
calleename,
callee
};
}
template<class T>
void emit_log(LPCTSTR& sep, std::ostream& os, T&& x)
{
os << sep << x;
sep = ",";
}
template<class Ret>
struct error_with_value
{
DWORD error;
Ret value;
bool has_error() const { return error != 0; }
friend std::ostream& operator<<(std::ostream& os, const error_with_value& ewv)
{
os << "{ error: " << ewv.error << ", value: ";
LPCTSTR sep = "";
emit_log(sep, os, ewv.value);
os << " }";
return os;
}
};
#define api(a) make_api_call_site(__FILE__, __LINE__, __func__, #a, a)
// this will need some specialisations...
void emit_log(LPCTSTR& sep, std::ostream& os, std::nullptr_t)
{
os << sep << "nullptr";
sep = ",";
}
template<class Ret, class...Args, class...GivenArgs>
auto api_call(api_call_site<Ret, Args...> const& callsite, GivenArgs&&...args) -> error_with_value<Ret>
{
// log call here
std::clog << callsite.file << ":" << callsite.line << "@" << callsite.current_function << " - ";
std::clog << "calling " << callsite.called_function << "(";
// appropriate code to print arguments in a safe way here...
LPCTSTR sep = "";
using expand = int[];
void(expand{0,
(emit_log(sep, std::clog, args),0)...
});
std::clog << ")";
error_with_value<Ret> result
{
0,
callsite.function(std::forward<GivenArgs>(args)...)
};
result.error = GetLastError();
std::clog << " -> returns: " << result;
return result;
}
int main()
{
char buffer[255];
DWORD bufsize = 255;
auto result = api_call(api(GetModuleFileName), nullptr, buffer, bufsize);
if (! result.has_error())
{
//
}
}
示例输出：
main.cpp:120@main - calling GetModuleFileName(nullptr,,255) -> returns: { error: 0, value: 14 }
http://coliru.stacked-crooked.com/a/e5da55af212d5500
如何获取API调用中的参数数量？
template<class Ret, class...Args>
struct api_call_site
{
const char* file;
int line;
const char* current_function;
const char* called_function;
Ret (* function)(Args...);
// like this
static constexpr std::size_t nofArgs()
{
return sizeof...(Args);
}
};
答案 1 :(得分：0)
感谢Richard Hodges，我可以解决我的问题：
现在每个api都有一个api_call，api_call后面的模板决定了被叫api的签名。比我要求的要好得多。
#include <windows.h>
#include <atlstr.h>
#include <tuple>
template <int ... Ns> struct seq_3 {};
template <int ... Ns> struct seq3_n {};
template <int I, int ... Ns> struct seq3_n<I, Ns...> {
using type = typename seq3_n<I - 1, I - 1, Ns...>::type;
};
template <int ... Ns> struct seq3_n<1, Ns...> {
// this sequence is more complicated in my real code, because
// there are more variables for logging, but not for api calling
using type = seq_3<Ns...>;
};
template <int N>
using seq3 = typename seq3_n<N>::type;
template <int ... Ms> struct seq_1 {};
template <int ... Ms> struct seq1_n {};
template <int J, int ... Ms> struct seq1_n<J, Ms...> {
using type = typename seq1_n<J - 1, J - 1, Ms...>::type;
};
template <int ... Ms> struct seq1_n<0, Ms...> {
using type = seq_1<Ms...>;
};
template <int M>
using seq1 = typename seq1_n<M>::type;
// according to the solution from Richard Hodges
// *********************************************
template<typename Ret, typename...Args>
struct api_call_site
{
const CString file;
int line;
const CString Caller;
const CString f_name;
Ret(WINAPI* function)(Args...);
static constexpr std::size_t nofArgs() {
return sizeof...(Args);
}
};
template<typename Ret, typename...Args>
auto make_api_call_site(const CString file, int line, const CString Caller, const CString f_name, Ret(WINAPI* callee)(Args...))
// WINAPI see also here https://stackoverflow.com/questions/18912931/why-need-to-use-winapi-for-the-syntax-for-declaring-function-pointers-for-fun
{
return api_call_site<Ret, Args...>
{
file,
line,
Caller,
f_name,
callee
};
}
template <typename Ret, typename...Args, typename TUP, int...INDICES3, int...INDICES1>
int fn(api_call_site<Ret, Args...> const& callsite, TUP tup, seq_3<INDICES3...>, seq_1<INDICES1...>) {
int err = 0;
// handling of return value from api call goes always in position 0 from tuple
std::get<0>(tup) = callsite.function(std::get<INDICES3>(tup) ...);
err = GetLastError();
/* calling next function (variadic too) with same tupel, but other sequence
myOpenDebugOutputString(project, file, line, Caller, f_name, std::get<INDICES1>(tup) ..., "stop");
*/
return err;
}
template<typename Ret, typename...Args, typename...GivenArgs>
int api_call(api_call_site<Ret, Args...> const& callsite, GivenArgs&&...args)
{
int err;
err = fn(callsite, std::tuple<GivenArgs...>(args...), seq3 <callsite.nofArgs()+1> {}, seq1 <sizeof...(GivenArgs)> {});
return err;
}
int main() {
DWORD size_path = 20; // make it small and get error 122
wchar_t path[MAX_PATH]; // ERROR_INSUFFICIENT_BUFFER
DWORD rval = 0;
int err = 0;
CString tolog1(L"EXE-Path determined");
int tolog2 = 25;
// old way without logging information
rval = GetModuleFileName(nullptr, path, MAX_PATH);
err = GetLastError();
// new way with logging any variables ... behind the must variables for the api
// ****************************************************************************
#define api(a) make_api_call_site(__FILE__, __LINE__, __func__, L#a, a)
err = api_call(api(GetModuleFileName), rval, nullptr, path, size_path, tolog1, tolog2);
return 0;
}