I'm calling a function with __func__ as one of the parameters so I can store the name of the function calling that function without having to retype the name of the calling function.
Is there a way of also getting the address of the calling function - a sort of getaddress(__func __) ??
The address will not tell you much; may I instead suggest the predefined macros __FILE__ and __LINE__? They will tell you the current source file (as a char*) and the current line. When you call a function passing those as parameters, you'd know which file/line was the function called from.
They're evaluated by the compiler at the point of usage. If you use them inside the function as opposed to passing them as parameters, they'll tell you the file/line of the function itself.
You can declare function pointers and then assign specific functions to them. So then you could have an array or other collection of such pointers as your table.
If this works for you and you accept the answer, I'm willing to come back and create a short summary here. (I know I'm not supposed to answer with just-a-link.) But see if this approach will do what you want.
https://www.learncpp.com/cpp-tutorial/78-function-pointers/
There is no portable standard C++ way to do this. You could use compiler/OS specific hacks, the same was as in C, but it's less useful in C++ since it only works on extern "C" names (where the __FUNCTION__/__func__ match what dlsym expects).
Since that hack only works on extern "C" names, this means you can't use it for templated functions, class methods, or function overloading (same function name, different argument prototypes), which is fairly restrictive. Mind you, even typing out the function name wouldn't work in some of those cases already (e.g. the name alone doesn't describe the prototype, so function overloading wouldn't work).
I think you need to rethink a design that's essentially demanding reflection in C++; the nature of optimizing compilers is that most of the functions you define don't actually have to exist, or if they do, they exist only with names that are meaningless to anything but the compiler. When you ask to dynamically determine the address of the current function from within the function, you're assuming the function exists, with an actual address, in the final compiled binary, when it could just as easily have been inlined into each actual call site, partially merged with a near identical function, etc.
Related
Right now, I am trying to call a function in C++ through a Json object. The Json object would provide me with the name of the callee function and all the parameters. I will be able to extract the parameters using a for loop, but I am not sure how I can pass them in. For loop only allows me to pass arguments one by one, and I did not find a way to call a function besides passing in all the arguments at once.
I've made a temporary solution of:
if (parameter_count == 1)
func(param_1);
if (parameter_count == 2)
func(param_1, param_2);
...
This solution seems would not work for all cases since it can only work for functions with a limited number of arguments (depending on how many ifs I write). Is there a better way for this? Thanks!
EDIT: Sorry if I was being unclear. I do not know anything about func. I will be reading func from DLL based on its string name. Since I can't really change the function itself, I wouldn't be able to pass in a vector or struct directly.
Or perhaps did I have the wrong understanding? Are we allowed to pass in a single vector in place of a lot of parameters?
Sorry for making a mess through so many edits on this question. Brandon's solution with libffi works. Thanks!
So the problem as I understand it is that you have a void * pointer (which would come from your platform's DLL loading code) which "secretly" is a pointer to a function with a signature which is only known at runtime. You'd like to call this function at runtime with specified arguments.
Unfortunately, this is not possible to do cleanly with standard C++ alone. C++ cannot work with types that are not present in the program at compile-time, and since there is an infinite number of potential function signatures involved here there is no way to compile them all in.
What you'll want to do instead is manually set up the stack frame on your call stack and then jump to it, either via inline assembly or via some library or compiler extension that accomplishes this for your platform.
Here is a simple example of doing this via inline assembly. (To do this in general you will need to learn your platform's calling convention in detail, and needless to say this will constrain your program to the platform(s) you've implemented this for.)
I haven't actually tried it, but gcc has a compiler extension __builtin_apply that is apparently just meant to forward the arguments from one method wholesale to another but which could perhaps be used to accomplish something like this if you learned the (apparently opaque) description of the method.
[Update: Apparently I missed this in the comments, but Brandon mentioned libffi, a library which implements a bunch of platforms' calling conventions. This sounds like it might be the best option if you want to take this sort of approach.]
A final option would be to constrain the allowed signatures of your functions to a specified list, e.g. something like
switch(mySignature)
{
case VOID_VOID:
dynamic_cast<std::function<void(void)> *>(myPtr)();
break;
case VOID_INT:
dynamic_cast<std::function<void(int)> *>(myPtr)(my_int_arg_1);
break;
// ...
}
(Syntax of the above may not be 100% correct; I haven't tested it yet.) Whether this approach is sensible for your purposes depends on what you're doing.
In C++ it is possible to declare that a function is const, which means, as far as I understand, that the compiler ensures the function does not modify the object. Is there something analogous in C++ where I can require that a function is pure? If not in C++, is there a language where one can make this requirement?
If this is not possible, why is it possible to require functions to be const but not require them to be pure? What makes these requirements different?
For clarity, by pure I want there to be no side effects and no use of variables other than those passed into the function. As a result there should be no file reading or system calls etc.
Here is a clearer definition of side effects:
No modification to files on the computer that the program is run on and no modification to variables with scope outside the function. No information is used to compute the function other than variables passed into it. Running the function should return the same thing every time it is run.
NOTE: I did some more research and encountered pure script
(Thanks for jarod42's comment)
Based on a quick read of the wikipedia article I am under the impression you can require functions be pure in pure script, however I am not completely sure.
Short answer: No. There is no equivalent keyword called pure that constrains a function like const does.
However, if you have a specific global variable you'd like to remain untouched, you do have the option of static type myVar. This will require that only functions in that file will be able to use it, and nothing outside of that file. That means any function outside that file will be constrained to leave it alone.
As to "side effects", I will break each of them down so you know what options you have:
No modification to files on the computer that the program is run on.
You can't constrain a function to do this that I'm aware. C++ just doesn't offer a way to constrain a function like this. You can, however, design a function to not modify any files, if you like.
No modification to variables with scope outside the function.
Globals are the only variables you can modify outside a function's scope that I'm aware of, besides anything passed by pointer or reference as a parameter. Globals have the option of being constant or static, which will keep you from modifying them, but, beyond that, there's really nothing you can do that I'm aware.
No information is used to compute the function other than variables passed into it.
Again, you can't constrain it to do so that I'm aware. However, you can design the function to work like this if you want.
Running the function should return the same thing every time it is run.
I'm not sure I understand why you want to constrain a function like this, but no. Not that I'm aware. Again, you can design it like this if you like, though.
As to why C++ doesn't offer an option like this? I'm guessing reusability. It appears that you have a specific list of things you don't want your function to do. However, the likelihood that a lot of other C++ users as a whole will need this particular set of constraints often is very small. Maybe they need one or two at a time, but not all at once. It doesn't seem like it would be worth the trouble to add it.
The same, however, cannot be said about const. const is used all the time, especially in parameter lists. This is to keep data from getting modified if it's passed by reference, or something. Thus, the compiler needs to know what functions modify the object. It uses const in the function declaration to keep track of this. Otherwise, it would have no way of knowing. However, with using const, it's quite simple. It can just constrain the object to only use functions that guarantee that it remains constant, or uses the const keyword in the declaration if the function.
Thus, const get's a lot of reuse.
Currently, C++ does not have a mechanism to ensure that a function has "no side effects and no use of variables other than those passed into the function." You can only force yourself to write pure functions, as mentioned by Jack Bashford. The compiler can't check this for you.
There is a proposal (N3744 Proposing [[pure]]). Here you can see that GCC and Clang already support __attribute__((pure)). Maybe it will be standardized in some form in the future revisions of C++.
In C++ it is possible to declare that a function is const, which means, as far as I understand, that the compiler ensures the function does not modify the object.
Not quite. The compiler will allow the object to be modified by (potentially ill-advised) use of const_cast. So the compiler only ensures that the function does not accidentally modify the object.
What makes these requirements [constant and pure] different?
They are different because one affects correct functionality while the other does not.
Suppose C is a container and you are iterating over its contents. At some point within the loop, perhaps you need to call a function that takes C as a parameter. If that function were to clear() the container, your loop will likely crash. Sure, you could build a loop that can handle that, but the point is that there are times when a caller needs assurance that the rug will not be pulled out from under it. Hence the ability to mark things const. If you pass C as a constant reference to a function, that function is promising to not modify C. This promise provides the needed assurance (even though, as I mentioned above, the promise can be broken).
I am not aware of a case where use of a non-pure function could similarly cause a program to crash. If there is no use for something, why complicate the language with it? If you can come up with a good use-case, maybe it is something to consider for a future revision of the language.
(Knowing that a function is pure could help a compiler optimize code. As far as I know, it's been left up to each compiler to define how to flag that, as it does not affect functionality.)
Is there any way a C++ beginner can implement something like this? For example:
./timerprogram sortalgorithm.cpp
where timerprogram.cpp at some point does something like argv[1](); to run the function whose name is given by the command-line argument?
Assuming that sortalgorithm.cpp was self-contained and had an array to sort already. I don't need the timing part, just how to call as a function a command-line argument. Is there anything build-in to C++ that will allow me to do this?
No. The answer is no.
Most of the stuff you see about this are inside jokes.
There are silly ways to make it look like its working, but they are silly, and certainly not for beginners.
Function names are used mostly by the compiler, to compile the code, and figure out when something calls a function "where" it actually is. Also by the linker too, but that's beside the point.
Although some C++ implementations might provide run-time extensions or libraries that can be used to resolve an address given its symbol name, the easiest and the most portable solution is for your program to simply have an array of strings, with your function names, and a pointer to the corresponding function.
Then, your main() searches the array for the requested function name, and invokes it via its function pointer.
How to implement this simple solution is going to be your homework assignment.
I'm experimenting with variable arguments in C++, using va_args. The idea is useful, and is indeed something I've used a lot in C# via the params functionality. One thing that frustrates me is the following excerpt regarding va_args, above:
Notice also that va_arg does not determine either whether the retrieved argument is the last argument passed to the function (or even if it is an element past the end of that list).
I find it hard to believe that there is no way to programmatically determine the number of variable arguments passed to the function from within that function itself. I would like to perform something like the following:
void fcn(int arg1 ...)
{
va_list argList;
va_start(argList, arg1);
int numRemainingParams = //function that returns number of remaining parameters
for (int i=0; i<numRemainingParams; ++i)
{
//do stuff with params
}
va_end(argList);
}
To reiterate, the documentation above suggests that va_arg doesn't determine whether the retrieved arg is the last in the list. But I feel this information must be accessible in some manner.
Is there a standard way of achieving this?
I find it hard to believe that there is no way to programmatically determine the number of variable arguments passed to the function from within that function itself.
Nonetheless, it is true. C/C++ do not put markers on the end of the argument list, so the called function really does not know how many arguments it is receiving. If you need to mark the end of the arguments, you must do so yourself by putting some kind of marker at the end of the list.
The called function also has no idea of the types or sizes of the arguments provided. That's why printf and friends force you to specify the precise datatype of the value to interpolate into the format string, and also why you can crash a program by calling printf with a bad format string.
Note that parameter passing is specified by the ABI for a particular platform, not by the C++/C standards. However, the ABI must allow the C++/C standards to be implementable. For example, an ABI might want to pass parameters in registers for efficiency, but it might not be possible to implement va_args easily in that case. So it's possible that arguments are also shadowed on the stack. In almost no case is the stack marked to show the end of the argument list, though, since the C++/C standards don't require this information to be made available, and it would therefore be unnecessary overhead.
The way variable arguments work in C and C++ is relatively simple: the arguments are just pushed on the stack and it is the callee's responsibility to somewhat figure out what arguments there are. There is nothing in the standard which provides a way to determine the number of arguments. As a result, the number of arguments are determined by some context information, e.g., the number of elements referenced in a format string.
Individual compilers may know how many elements there are but there is no standard interface to obtain this value.
What you could do instead, however, is to use variadic templates: you can determine very detailed information on the arguments being passed to the function. The interface looks different and it may be necessary to channel the arguments into some sort of data structure but on the upside it would also work with types you cannot pass using variable arguments.
No, there isn't. That's why variable arguments are not safe. They're a part of C, which lacks the expressiveness to achieve type safety for "convenient" variadic functions. You have to live with the fact that C contains constructions whose very correctness depends on values and not just on types. That's why it is an "unsafe language".
Don't use variable arguments in C++. It is a much stronger language that allows you to write equally convenient code that is safe.
No, there's no such way. If you have such a need, it's probably best to pack those function parameters in a std::vector or a similar collection which can be iterated.
The variable argument list is a very old concept inherited from the C history of C++. It dates back to the time where C programmers usually had the generated assembler code in mind.
At that time the compiler did not check at all if the data you passed to a function when calling it matched the data types the function expected to receive. It was the programmer's responsibility to do that right. If, for example, the caller called the function with a char and the function expected an int the program crashed, although the compiler didn't complain.
Today's type checking prevents these errors, but with a variable argument list you go back to those old concepts including all risks. So, don't use it if you can avoid it somehow.
The fact that this concept is several decades old is probably the reason that it feels wrong compared to modern concepts of safe code.
Is there a way to determine how many parameters a Lua function takes just before calling it from C/C++ code?
I looked at lua_Debug and lua_getinfo but they don't appear to provide what I need.
It may seem a bit like I am going against the spirit of Lua but I really want to bullet proof the interface that I have between Lua and C++. When a C++ function is called from Lua code the interface verifies that Lua has supplied the correct number of arguments and the type of each argument is correct. If a problem is found with the arguments a lua_error is issued.
I'd like to have similar error checking the other way around. When C++ calls a Lua function it should at least check that the Lua function doesn't declare more parameters than are necessary.
What you're asking for isn't possible in Lua.
You can define a Lua function with a set of arguments like this:
function f(a, b, c)
body
end
However, Lua imposes no restrictions on the number of arguments you pass to this function.
This is valid:
f(1,2,3,4,5)
The extra parameters are ignored.
This is also valid:
f(1)
The remaining arguments are assigned 'nil'.
Finally, you can defined a function that takes a variable number of arguments:
function f(a, ...)
At which point you can pass any number of arguments to the function.
See section 2.5.9 of the Lua reference manual.
The best you can do here is to add checks to your Lua functions to verify you receive the arguments you expect.
You can determine the number of parameters, upvalues and whether the function accepts variable number of arguments in Lua 5.2, by using the 'u' type to fill nups, nparams, isvararg fields by get_info(). This feature is not available in Lua 5.1.
I wouldn't do this on the Lua side unless you're in full control of Lua code you're validating. It is rather common for Lua functions to ignore extra arguments simply by omitting them.
One example is when we do not want to implement some methods, and use a stub function:
function do_nothing() end
full_api = {}
function full_api:callback(a1, a2) print(a1, a2) end
lazy_impl = {}
lazy_impl.callback = do_nothing
This allows to save typing (and a bit of performance) by reusing available functions.
If you still want to do function argument validation, you have to statically analyze the code. One tool to do this is Metalua.
No, not within standard Lua. And is Aaron Saarela is saying, it is somewhat outside the spirit of Lua as I understand it. The Lua way would be to make sure that the function itself treats nil as a sensible default (or converts it to a sensible default with something like name = name or "Bruce" before its first use) or if there is no sensible default the function should either throw an error or return a failure (if not name then error"Name required" end is a common idiom for the former, and if not name then return nil, "name required" end is a common idiom for the latter). By making the Lua side responsible for its own argument checks, you get that benefit regardless of whether the function is called from Lua or C.
That said, it is possible that your modules could maintain an attribute table indexed by function that contains the info you need to know. It would require maintenance, of course. It is also possible that MetaLua could be used to add some syntax sugar to create the table directly from function declarations at compile time. Before calling the Lua function, you would use it directly to look up any available attributes and use them to validate the call.
If you are concerned about bullet-proofing, you might want to control the function environment to use some care with what (if any) globals are available to the Lua side, and use lua_pcall() rather than lua_call() so that you catch any thrown errors.
The information you ask for is not available in all cases. For example, a Lua function might actually be implemented in C as a lua_CFunction. From Lua code there is no way to distinguish a pure Lua function from a lua_CFunction. And in the case of a lua_CFunction, the number of parameters is not exposed at all, since it's entirely dependent on the way the function is implemented.
On the other hand, what you can do is provide a system for functions writers (be it in pure Lua or in C) to advertise how many parameters their functions expect. After creating the function (function f(a, b, c) end) they would simply pass it to a global function (register(f, 3)). You would then be able to retrieve that information from your C++ code, and if the function didn't advertise its parameters then fallback to what you have now. With such a system you could even advertise the type expected by the parameters.