I have a function with following signature
char requestApiCall(int num, const wchar_t* pParams = 0, ...)
{
...
}
Now I want to to get all pParams in an array (or to be able to iterate over it). I know this is possible with some macros, but I have no idea how to do it.
Any help would be appreciated.
P.S. I'm using MinGW if it matters.
UPDATE
my question caused confusion. I will try to clarify (sorry for my grammar). Both Object Pascal and C# has the ability to pass unlimited amount of parameters to a method. In C# we achieve this with params keyword:
void Foo(params string[] strs)
{
...
}
Foo("first", "second", "another one", "etc");
I want to achieve same result in C++ without using any object/class. In my case, type safety is not a concern, but if there is a type safe way to achieve that goal, I will gladly hear your comments :)
Thanks
You need to look at the functions and macros declared in stdarg.h. Here is a tutorial that explains it.
http://publications.gbdirect.co.uk/c_book/chapter9/stdarg.html
I'm not sure what your function parameters are supposed to represent but I think you'll find that it needs to change.
By the way, I find that for C++ I can usually avoid variadic functions. This has the advantage of preserving type safety. Are you sure you really need a variadic function?
Using variadic function arguments is a dangerous and tricky business, and almost surely there is a better way - for example, you might pass an std::vector<std::wstring>& to your function!
OK, that said, here's how to use variadic arguments. The key point is that it is your responsibility to know the number and types of the arguments!
#include <cstdarg>
char requestApiCall(int num, const wchar_t* pParams, ...)
{
va_list ap; // the argument pointer
va_start(ap, pParams); // initialize it with the right-most named parameter
/** Perform magic -- YOU have to know how many arguments you are getting! **/
int a = va_arg(ap, int); // extract one int
double d = va_arg(ap, double) // one double
char * s = va_arg(ap, char*) // one char*
/* ... and so forth ... */
va_end(ap); // all done, clean up
}
Just for completeness, I would redefine the function as this:
char requestApiCall(std::vector<std::wstring> & params)
{
for (std::vector<std::wstring>::const_iterator it = params.begin(), end = params.end(); it != end; ++it)
{
// do something with *it
}
/* ... */
}
A good example of what you are trying to accomplish is the exec family of functions. exec() takes an variable list of arguments all of which are expected to be const char*. The last item is a NULL ((char*)0). The last item is the indicator for when the list of items is complete.
You can use the variadic macros in stdargs.h as others have described.
Related
This question already has answers here:
Count number of parameters in C variable argument method call
(4 answers)
Closed 7 years ago.
So in my function I want to take a variable number of char pointers.
void myFunction(char *_Name, ...);
Only, when I iterate through the arguments my code breaks because I'm not sure how to stop on the last argument I pass and not keep going.
void myClass::myFunction(char *_Name, ...)
{
char *a;
std::vector<char*> names;
va_list VariablePathList;
va_start (VariablePathList, _Name);
a = _Name;
bool exitLoop = false;
while(!exitLoop)
{
names.emplace_back(a);
a = va_arg(VariablePathList, char *);
//what must I do to set "exitLoop = true" at the right time?
}
va_end (VariablePathList);
}
Say I pass two char pointers into myFunction, then the code breaks on the third loop when va_arg is calling on unreadable memory.
Without telling myFunction how many agruments I have given it how can it find out the number to set exitLoop before the code breaks?
I can not seem to find straight forward information on how printf() and the like do this, so there could lie the solution I'm after.
Really, thank you in advance.
printf does it by parsing the format string. Some other functions do it by accepting an explicit count argument before the varargs. Which way to go will depend on your specific needs.
Really, though, it's 2015 now so you should be using variadic templates rather than archaic C customs. I also definitely do not advise storing char* in containers.
Here is all you need:
#include <vector>
#include <string>
template <typename... T>
void foo(T... args)
{
std::vector<std::string> names{args...};
// now use `names`
}
(live demo)
I mean strings like "(%d%%%d) some text %s , %d%% %x %o %#x %#o \n". The printf family of functions somehow know how many args they would need from this string so separate argument for it is not needed. Is this functionality provided in c/c++ in terms of a separate function so I could write my own printf-like functions ? (yes I put these percents intentionally to accent how complicated it could get, so it is not simple counting of percent characters)
The problem with printf family is that it is not safe, and it doesn't really know or care how many format elements are there in string or in argument list.
printf family uses something called "variadic functions"; only one parameter is named (format string) and others are taken from the stack, without respecting their type or size - their type is deduced from format string, which makes it type-unsafe, and finds how many arguments to use by iterating over the string and finding all formatting specifiers, which makes it argument-number-unsafe. You can write variadic functions; syntax is
void foo(...);
and later you can use those macros and types, although if you are using C++, you should use C++11's variadic templates instead of C variadic functions, as variadic templates are type safe - you don't loose type information anywhere, and with universal references (but not quite thanks them) they are more powerful than variadic functions.
No, there is no standard library function for parsing printf-style format strings. If you want to write your own string formatting function, then I strongly recommend against following the printf example. It's inherently unsafe for the reasons mentioned in the others answers. It also has problems with localization since word order changes in some languages. Personally, I would write a type-safe function using templates and copying the .NET style of format strings. (Actually, I did do that--it's a lot of work but also fun.)
the printf family of functions only knows about one parameter, the first one...
It basically scans the string and each time it encounters a format specifier that it understands it pulls then next argument of that size from the argument list... this is an easily corruptible behavior.
imagine:
printf("%i",someInt); // fine
printf("%i",someLong); // depending on endianness and sized
// of those types could be the high or low 32 bits(probably)
printf("%i %i",someInt); // depending on abi could crash, read a int sized chunk
// of stack, or a register that would correspond to that
// parameter.
so inherently not safe..
you should pay attention to warnings and when writing portable code do things like:
size_t t = 5; //can be 32 or 64 bit depending on arch.
printf("%ull",(unsigned long long)t);
Edit. I guess I only half answered the question...
you can define your own variadic functions, the following adds count number of params and returns the result.
int sumList(int count, ...);
int main(int argc, const char * argv[])
{
printf("the answer is: %i",sumList(4,1,2,3,4));
return 0;
}
int sumList(int count, ...)
{
va_list args;
va_start(args, count);
int sum = 0;
for (int i = 0; i<count; i++) {
sum += va_arg(args, int);
}
va_end(args);
return sum;
}
I have been reading that some compilers support va_list with macros and users were able to overload the functionality with other macros in order to count the va_list.
With visual studio, is there a way to determine if the va_list is empty (aka count==0)? Basically I would like to know this condition:
extern void Foo(const char* psz, ...);
void Test()
{
Foo("My String"); // No params were passed
}
My initial thought was to do something like this:
va_list vaStart;
va_list vaEnd;
va_start(vaStart, psz);
va_end(vaEnd);
if (vaStart == vaEnd) ...
The problem is that va_end only sets the param to null.
#define _crt_va_start(ap,v) ( ap = (va_list)_ADDRESSOF(v) + _INTSIZEOF(v) )
#define _crt_va_arg(ap,t) ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) )
#define _crt_va_end(ap) ( ap = (va_list)0 )
I was thinking of maybe incorporating a terminator but I would want it to be hidden from the caller so that existing code doesnt need to be changed.
There is no way to tell how many arguments are passed through ..., nor what type they are. Variadic function parameters can only be used if you have some other way (e.g. a printf-style format string) to tell the function what to expect; and even then there is no way to validate the arguments.
C++11 provides type-safe variadic templates. I don't know whether your compiler supports these, or whether they would be appropriate for your problem.
I realize this question is fairly old, but I thought it might be helpful to flesh it out a bit. As Mike Seymour answered quite correctly, there is no absolutely reliable way to determine the number of arguments in a va_list. That's why the conventional way to define a variadic function is to include a parameter that has that information, like so: void func(const char *str, int count, ...);, which defines a contract your callers are supposed to abide by.
EDIT: The standard (7.16.1.1.3) is actually silent on the value returned by va_arg(vl, type) for any call past the end of the variable argument list. The most common case for most types is typed-zero. However, CAVEAT EMPTOR - it doesn't have to be.
The value returned from va_arg(vl, type) when there are no more arguments is a typed-zero value. For numeric types, it is 0. For pointers, it is a NULL pointer. For structs, it is a struct with all fields zeroed. If you elect to copy the va_list and try to count the copy, like so:
void func(const char *str, ...) {
va_list vl;
va_list vc;
int x;
int count;
count = 0;
va_start(vl, str);
va_copy(vc, vl);
do {
x = va_arg(vc, int);
if (x == 0) break;
count++;
} while (1)
va_end(vc);
.
.
. // do something, or something else,
. // based on the number of args in the list
.
va_end(vl);
You would have to make the assumption that the caller would abide by the contract not to pass a NULL or zero value in the list. Either way, you have to understand that the caller of a variadic function is responsible for abiding by the stated contract. So write your function, publish the contract, and sleep easy.
I am aware that my answer isn't an "orthodox" answer, however due to the limitation of the va_list macro, I found the following solution to work. We can look at a va_list as an array of chars, and even better, a null terminated one, so you can try
va_list argptr;
if(strcmp(argptr,""))
{
// not empty
}
else
{
// empty
}
I tried that and it worked for me.
I used Visual Studio 2013 Ultimate. The project type is Win32 Console Application. No compilation errors.
I have to classes, an Executer with these methods:
Executer()
struct Execute(string s)
Lookup(string name, int module, int num, ...)
and a Parser:
Parser()
struct Parse(string s)
The Exectuers Execute method calls the Parsers Parse method. The Parser then chucks the string into smaller bits (it explodes the string on the ;-sign) and returns a struct to the Execute method. This struct it uses to call the Lookup method.
The struct that the Parse returns holds some standard information:
An command name
A senderId (a username, a mac address and a password)
A variable number of arguments
And that is my problem. The Lookup method take variable arguments, but how do I handle the the hand over of these variable arguments by the struct? Im not an expert in C and C++. Should I mass the two classes togheter? So the Parser method could call the Execute method, sparing the struct away.
Or maybe there is a way of parsing an unknown variable of arguments at runtime? By some sort of array?
EDIT
I cant use the STL library from C++. I only use the C++ class and virtual feature. Im writing to an compiler where Im restricted to use almost all of the C libraries + the magic skills of C++ (virtual and class). SOory for not telling that right away.
EDIT 2
Im writing code to an embedded system and thereby using avr-gcc to compile my code. Thats why I cant use STL. The avr-gcc doesnt support this.
Use std::vector<> or a simular container that can hold an arbitrary number of entries.
struct {
std::string commandName;
sender_t senderId;
std::vector<arg_t> arguments;
};
Edit: oh, you can't use std::vector. In that case: use an array and store the length:
struct {
const char* commandName;
sender_t senderId;
int argumentCount;
int maxArgumentCount; // you might not need this
arg_t* arguments; // pointer to array of (at least) argumentCount elements.
};
Use malloc() or new() to create the array for the arguments.
I would suggest to wrap the argumentCount, maxArgumentCount and arguments in a separate class, which can handle the malloc/new and free/delete as well. This will make it easier to prevent memory leaks.
In the end, you'll have written your own vector_of_arg_t class, so maybe have a look at some basic vector implementation. There must be tutorials on that on the web.
You could declare your Lookup method as follows:
void Lookup(string name, int module, int num, std::vector<std::string> &args);
By storing the variable arguments in an args array, you can have as many as you want.
See
Q: How can I write a function which takes a variable number of arguments and passes them to some other function (which takes a variable number of arguments)?
A: In general, you cannot. Ideally, you should provide a version of that other function which accepts a va_list pointer.
Suppose you want to write a faterror function which will print a fatal error message, then exit. You might like to write it in terms of the error function of question 15.5:
void faterror(const char *fmt, ...)
{
error(fmt, what goes here? );
exit(EXIT_FAILURE);
}
but it's not obvious how to hand faterror's arguments off to error.
<snip>
Read on at
http://c-faq.com/varargs/handoff.html
Is it possible to have a function with variable arguments and no named argument?
For example:
SomeLogClass("Log Message Here %d").Log(5);
SomeLogClass("Log Message Here %d").Error(5);
Take a look at QString's arg methods. Those seem to be something you're looking for.
You can definitely roll your own, although implementation might turn out to be not really trivial, especially if you would like it to support printf format specifiers. If printf style is not necessary, chaining a replace_all kind of calls sounds doable.
Can you write code like that above - yes you can. But you cannot portable write a variadic function without at least one non-variadic parameter. In printf(), for example, this is the format string. In other words, you can write function s like:
int printf( const char * format, ... );
but not:
int printf( ... );
Where I am from we use this:
Log << "LogMessageHere: " << ErrorClass << 5 << whatever << std::endl;
It is not exactly an answer to you question, but it is a solution to your problem, and I think it is more c++ like.
The right answer is no, you can't define only variable arguments, because the mechanism in C/C++ to do so uses a fixed argument in order to compute an address, like this:
void f(int a, ...) {
va_list args;
va_start(args, a); // without a, this macro DOESN'T WORK!!!
}
The answer you flagged gets around it by defaulting the arguments. But what this should teach the newbies is that defaulting the arguments doesn't mean that arguments aren't passed, it means that you don't have to type them.
void f (int a = 0, ...)
So when you call f you can write:
f();
but internally, it's writing f(0)