在可变参数模板中扩展变量名称

Question

我不确定如何访问可变参数模板中的变量名称。

#define DebugVars(...) DEBUG_VARS(__FILE__, __LINE__, __FUNCTION__, ## __VA_ARGS__)
#define GetVarName(Variable) (#Variable)

void Log(const char* file, const int line, const char* func, const std::string& message)
{
    printf("file:%s, line:%d, func:%s \n%s", file, line, func, message.c_str());
}

template <typename... Args>
void DEBUG_VARS(const char* file, const int line, const char* func, Args&&... args)
{
    std::ostringstream ss;
    using expander = int[];
    (void) expander { 0, (void(ss << GetVarName(args) << ": " << args << "\n"), 0) ...}; 
    Log(file, line, func, ss.str());
}

void main()
{
    int number = 37;
    float pie = 3.14;
    std::string str = "test string";

    DebugVars(number, pie, str);
}

输出

file:main.cpp, line:29, func:main 
args: 37
args: 3.14
args: test string

预期输出

file:main.cpp, line:29, func:main 
number: 37
pir: 3.14
str: test string

Example

DebugVars(...) 很容易放入某处的函数中进行调试，但我需要变量名才能发挥作用。

Answer 1

最重要的是，您无法在 DEBUG_VARS 中获取变量名称，因为这些名称只是 DEBUG_VARS 函数中不存在的标识符。但是，如果您将宏更改为也将 __VA_ARGS__ 的字符串化版本与参数本身一起传递，则可以将它们标记化并在折叠表达式中使用另一个字符串流，以将它们打印出来...

#include <iostream>
#include <sstream>

#define DebugVars(...) DEBUG_VARS(__FILE__, __LINE__, __FUNCTION__, #__VA_ARGS__,__VA_ARGS__)

void Log(const char* file, const int line, const char* func, const std::string& message)
{
    printf("file:%s, line:%d, func:%s, message:%s \n", file, line, func, message.c_str());
}

template < typename... Args>
void DEBUG_VARS(const char* file, const int line, const char* func, const std::string& names, Args&&... args)
{

    std::stringstream names_ss;
    for (const char& c : names )
    {
        if (c == ','){
            names_ss << " ";
            continue;
        }
        names_ss << c;
    }

    std::string name;
    std::ostringstream ss;
    ss << "\n";
    using expander = int[];
    (void) expander { 0, (
          names_ss >> name, ss << name << ": " << args << "\n"
    ,0) ...};
    Log(file, line, func, ss.str());

}

int main()
{
    int number = 37;
    float pie = 3.14;
    std::string str = "test string";

    DebugVars(number, pie, str);
    return 0;
}

Demo

显然这只适用于命名参数。并且没有具有其他参数的嵌套函数调用，否则将需要进一步处理字符串。

Answer 2

来自foreach-macro-on-macros-arguments

#define PP_NARG(...)    PP_NARG_(__VA_ARGS__,PP_RSEQ_N())
#define PP_NARG_(...)   PP_ARG_N(__VA_ARGS__)

#define PP_ARG_N( \
        _1, _2, _3, _4, _5, _6, _7, _8, _9,_10,  \
        _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
        _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
        _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
        _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
        _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
        _61,_62,_63,N,...) N

#define PP_RSEQ_N() \
        63,62,61,60,                   \
        59,58,57,56,55,54,53,52,51,50, \
        49,48,47,46,45,44,43,42,41,40, \
        39,38,37,36,35,34,33,32,31,30, \
        29,28,27,26,25,24,23,22,21,20, \
        19,18,17,16,15,14,13,12,11,10, \
        9,8,7,6,5,4,3,2,1,0


/* need extra level to force extra eval */
#define Paste(a,b) a ## b
#define XPASTE(a,b) Paste(a,b)


/* APPLYXn variadic X-Macro by M Joshua Ryan      */
/* Free for all uses. Don't be a jerk.            */
/* I got bored after typing 15 of these.          */
/* You could keep going upto 64 (PPNARG's limit). */
#define APPLYX1(a)           X(a)
#define APPLYX2(a,b)         X(a) X(b)
#define APPLYX3(a,b,c)       X(a) X(b) X(c)
#define APPLYX4(a,b,c,d)     X(a) X(b) X(c) X(d)
#define APPLYX5(a,b,c,d,e)   X(a) X(b) X(c) X(d) X(e)
#define APPLYX6(a,b,c,d,e,f) X(a) X(b) X(c) X(d) X(e) X(f)
#define APPLYX7(a,b,c,d,e,f,g) \
    X(a) X(b) X(c) X(d) X(e) X(f) X(g)
#define APPLYX8(a,b,c,d,e,f,g,h) \
    X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h)
#define APPLYX9(a,b,c,d,e,f,g,h,i) \
    X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i)
#define APPLYX10(a,b,c,d,e,f,g,h,i,j) \
    X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j)
#define APPLYX11(a,b,c,d,e,f,g,h,i,j,k) \
    X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k)
#define APPLYX12(a,b,c,d,e,f,g,h,i,j,k,l) \
    X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k) X(l)
#define APPLYX13(a,b,c,d,e,f,g,h,i,j,k,l,m) \
    X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k) X(l) X(m)
#define APPLYX14(a,b,c,d,e,f,g,h,i,j,k,l,m,n) \
    X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k) X(l) X(m) X(n)
#define APPLYX15(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o) \
    X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k) X(l) X(m) X(n) X(o)
#define APPLYX_(M, ...) M(__VA_ARGS__)
#define APPLYXn(...) APPLYX_(XPASTE(APPLYX, PP_NARG(__VA_ARGS__)), __VA_ARGS__)

那么你可能会这样做：

#define X(n) , std::make_pair(#n, n)
#define DEBUG_VARS(...) Log(__FILE__, __LINE__, __FUNCTION__ APPLYXn(__VA_ARGS__))

有

template <typename... Args>
void Log(const char* file, int line, const char* func, Args&&... args)
{
    std::cout << "file:" << file << ", line:" << line << ", func:" << func << " \n";

    ((std::cout << args.first << ": " << args.second << std::endl), ...);

}

Demo

在可变参数模板中扩展变量名称

Expanding variable names in a variadic template

c++

variadic

expander

variable-names

variadic-templates