In my code, I have repeatedly this expression:
T foo;
do_sth(foo, "foo");
I am considering stringifying the variable name, like this:
#define VARNAME(Var) (#Var)
void do_sth_new(T foo) { do_sth(foo, VARNAME(foo)); };
T foo;
do_sth_new(foo);
Is it good practice? Is there any better alternative in C++11?
As you show it, it doesn't work since VARNAME(foo) will always be "foo" (as this is the parameter's name). You have to write do_sth_new itself as macro:
#define do_sth_new(_foo) \
do { do_sth(_foo, #_foo); } while (false)
Only then will this:
T bar;
do_sth_new(bar);
generate "bar".
And no, there is no alternative to using the preprocessor since this is an operation on the lexical level. You'd need LISP-level modification of the AST in the language to have a better solution, which is unlikely to ever happen.
Sadly, no. There is still no solution (not even in C++17) to this problem. There might be something once static reflection will be added to C++. But for the time being you're stuck with the macro.
There is no real way to avoid the macro to do the stringify.
What you can do is dress it in a more c++ object-oriented way, especially if you want to do multiple different methods that take an object and its var name, and if this is a debug feature you might want to disable in production.
So I'm proposing you declare a template class DebugContextWrap, and objects of this type (or const ref) can be passed into the function as a single parameter, instead of having 2 parameters.
The one downside is that where your function code actually wants to access the actual value then you would have to perform an indirection through operator -> or data() as you do for iterators.
You could then write a macro that generates instances of DebugContextWrap - something like:
template class FooType
class DebugContextWrap
{
FooType& fooVal;
const char* debugName;
const char* debug__FILE__val;
const int debug__LINE__val;
public:
DebugContextWrap(FooType& fooVal,
const char* debugName, const char* debug__FILE__val, const int debug__LINE__val)
{ ... }
DebugContextWrap(FooType& fooVal) // implicit when caller doesn't use DEBUG_WRAP
{ ... }
FooType* operator ->()const
{ return &foo; }
FooType& operator *()const
{ return foo; }
FooType& Data()const
{ return foo; }
const char* DebugName()const
{ return debugName; }
};
#define DEBUG_WRAP(foo) \
DebugContextWrap<decltype foo>(foo, #foo, __FILE__, __LINE__)
void do_sth(const DebugContextWrap<FooType>& foo);
do_sth(DEBUG_WRAP(foovar));
Related
The Goal:
decide during runtime which templated function to use and then use it later without needing the type information.
A Partial Solution:
for functions where the parameter itself is not templated we can do:
int (*func_ptr)(void*) = &my_templated_func<type_a,type_b>;
this line of code can be modified for use in an if statement with different types for type_a and type_b thus giving us a templated function whose types are determined during runtime:
int (*func_ptr)(void*) = NULL;
if (/* case 1*/)
func_ptr = &my_templated_func<int, float>;
else
func_ptr = &my_templated_func<float, float>;
The Remaining Problem:
How do I do this when the parameter is a templated pointer?
for example, this is something along the lines of what I would like to do:
int (*func_ptr)(templated_struct<type_a,type_b>*); // This won't work cause I don't know type_a or type_b yet
if (/* case 1 */) {
func_ptr = &my_templated_func<int,float>;
arg = calloc(sizeof(templated_struct<int,float>, 1);
}
else {
func_ptr = &my_templated_func<float,float>;
arg = calloc(sizeof(templated_struct<float,float>, 1);
}
func_ptr(arg);
except I would like type_a, and type_b to be determined during runtime. I see to parts to the problem.
What is the function pointers type?
How do I call this function?
I think I have the answer for (2): simply cast the parameter to void* and the template function should do an implicit cast using the function definition (lease correct me if this won't work as I think it will).
(1) is where I am getting stuck since the function pointer must include the parameter types. This is different from the partial solution because for the function pointer definition we were able to "ignore" the template aspect of the function since all we really need is the address of the function.
Alternatively there might be a much better way to accomplish my goal and if so I am all ears.
Thanks to the answer by #Jeffrey I was able to come up with this short example of what I am trying to accomplish:
template <typename A, typename B>
struct args_st {
A argA;
B argB;
}
template<typename A, typename B>
void f(struct args_st<A,B> *args) {}
template<typename A, typename B>
void g(struct args_st<A,B> *args) {}
int someFunction() {
void *args;
// someType needs to know that an args_st struct is going to be passed
// in but doesn't need to know the type of A or B those are compiled
// into the function and with this code, A and B are guaranteed to match
// between the function and argument.
someType func_ptr;
if (/* some runtime condition */) {
args = calloc(sizeof(struct args_st<int,float>), 1);
f((struct args_st<int,float> *) args); // this works
func_ptr = &g<int,float>; // func_ptr should know that it takes an argument of struct args_st<int,float>
}
else {
args = calloc(sizeof(struct args_st<float,float>), 1);
f((struct args_st<float,float> *) args); // this also works
func_ptr = &g<float,float>; // func_ptr should know that it takes an argument of struct args_st<float,float>
}
/* other code that does stuff with args */
// note that I could do another if statement here to decide which
// version of g to use (like I did for f) I am just trying to figure out
// a way to avoid that because the if statement could have a lot of
// different cases similarly I would like to be able to just write one
// line of code that calls f because that could eliminate many lines of
// (sort of) duplicate code
func_ptr(args);
return 0; // Arbitrary value
}
Can't you use a std::function, and use lambdas to capture everything you need? It doesn't appear that your functions take parameters, so this would work.
ie
std::function<void()> callIt;
if(/*case 1*/)
{
callIt = [](){ myTemplatedFunction<int, int>(); }
}
else
{
callIt = []() {myTemplatedFunction<float, float>(); }
}
callIt();
If I understand correctly, What you want to do boils down to:
template<typename T>
void f(T)
{
}
int somewhere()
{
someType func_ptr;
int arg = 0;
if (/* something known at runtime */)
{
func_ptr = &f<float>;
}
else
{
func_ptr = &f<int>;
}
func_ptr(arg);
}
You cannot do that in C++. C++ is statically typed, the template types are all resolved at compile time. If a construct allowed you to do this, the compiler could not know which templates must be instanciated with which types.
The alternatives are:
inheritance for runtime polymorphism
C-style void* everywhere if you want to deal yourself with the underlying types
Edit:
Reading the edited question:
func_ptr should know that it takes an argument of struct args_st<float,float>
func_ptr should know that it takes an argument of struct args_st<int,float>
Those are incompatible. The way this is done in C++ is by typing func_ptr accordingly to the types it takes. It cannot be both/all/any.
If there existed a type for func_ptr so that it could take arguments of arbitrary types, then you could pass it around between functions and compilation units and your language would suddenly not be statically typed. You'd end up with Python ;-p
Maybe you want something like this:
#include <iostream>
template <typename T>
void foo(const T& t) {
std::cout << "foo";
}
template <typename T>
void bar(const T& t) {
std::cout << "bar";
}
template <typename T>
using f_ptr = void (*)(const T&);
int main() {
f_ptr<int> a = &bar<int>;
f_ptr<double> b = &foo<double>;
a(1);
b(4.2);
}
Functions taking different parameters are of different type, hence you cannot have a f_ptr<int> point to bar<double>. Otherwise, functions you get from instantiating a function template can be stored in function pointers just like other functions, eg you can have a f_ptr<int> holding either &foo<int> or &bar<int>.
Disclaimer: I have already provided an answer that directly addresses the question. In this answer, I would like to side-step the question and render it moot.
As a rule of thumb, the following code structure is an inferior design in most procedural languages (not just C++).
if ( conditionA ) {
// Do task 1A
}
else {
// Do task 1B
}
// Do common tasks
if ( conditionA ) {
// Do task 2A
}
else {
// Do task 2B
}
You seem to have recognized the drawbacks in this design, as you are trying to eliminate the need for a second if-else in someFunction(). However, your solution is not as clean as it could be.
It is usually better (for code readability and maintainability) to move the common tasks to a separate function, rather than trying to do everything in one function. This gives a code structure more like the following, where the common tasks have been moved to the function foo().
if ( conditionA ) {
// Do task 1A
foo( /* arguments might be needed */ );
// Do task 2A
}
else {
// Do task 1B
foo( /* arguments might be needed */ );
// Do task 2B
}
As a demonstration of the utility of this rule of thumb, let's apply it to someFunction(). ... and eliminate the need for dynamic memory allocation ... and a bit of cleanup ... unfortunately, addressing that nasty void* is out-of-scope ... I'll leave it up to the reader to evaluate the end result. The one feature I will point out is that there is no longer a reason to consider storing a "generic templated function pointer", rendering the asked question moot.
// Ideally, the parameter's type would not be `void*`.
// I leave that for a future refinement.
void foo(void * args) {
/* other code that does stuff with args */
}
int someFunction(bool condition) {
if (/* some runtime condition */) {
args_st<int,float> args;
foo(&args);
f(&args); // Next step: pass by reference instead of passing a pointer
}
else {
args_st<float,float> args;
foo(&args);
f(&args); // Next step: pass by reference instead of passing a pointer
}
return 0;
}
Your choice of manual memory management and over-use of the keyword struct suggests you come from a C background and have not yet really converted to C++ programming. As a result, there are many areas for improvement, and you might find that your current approach should be tossed. However, that is a future step. There is a learning process involved, and incremental improvements to your current code is one way to get there.
First, I'd like to get rid of the C-style memory management. Most of the time, using calloc in C++ code is wrong. Let's replace the raw pointer with a smart pointer. A shared_ptr looks like it will help the process along.
// Instead of a raw pointer to void, use a smart pointer to void.
std::shared_ptr<void> args;
// Use C++ memory management, not calloc.
args = std::make_shared<args_st<int,float>>();
// or
args = std::make_shared<args_st<float,float>>();
This is still not great, as it still uses a pointer to void, which is rarely needed in C++ code unless interfacing with a library written in C. It is, though, an improvement. One side effect of using a pointer to void is the need for casts to get back to the original type. This should be avoided. I can address this in your code by defining correctly-typed variables inside the if statement. The args variable will still be used to hold your pointer once the correctly-typed variables go out of scope.
More improvements along this vein can come later.
The key improvement I would make is to use the functional std::function instead of a function pointer. A std::function is a generalization of a function pointer, able to do more albeit with more overhead. The overhead is warranted here in the interest of robust code.
An advantage of std::function is that the parameter to g() does not need to be known by the code that invokes the std::function. The old style of doing this was std::bind, but lambdas provide a more readable approach. Not only do you not have to worry about the type of args when it comes time to call your function, you don't even need to worry about args.
int someFunction() {
// Use a smart pointer so you do not have to worry about releasing the memory.
std::shared_ptr<void> args;
// Use a functional as a more convenient alternative to a function pointer.
// Note the lack of parameters (nothing inside the parentheses).
std::function<void()> func;
if ( /* some runtime condition */ ) {
// Start with a pointer to something other than void.
auto real_args = std::make_shared<args_st<int,float>>();
// An immediate function call:
f(real_args.get());
// Choosing a function to be called later:
// Note that this captures a pointer to the data, not a copy of the data.
// Hence changes to the data will be reflected when this is invoked.
func = [real_args]() { g(real_args.get()); };
// It's only here, as real_args is about to go out of scope, where
// we lose the type information.
args = real_args;
}
else {
// Similar to the above, so I'll reduce the commentary.
auto real_args = std::make_shared<args_st<float,float>>();
func = [real_args]() { g(real_args.get()); };
args = real_args;
}
/* other code that does stuff with args */
/* This code is probably poor C++ style, but that can be addressed later. */
// Invoke the function.
func();
return 0;
}
Your next step probably should be to do some reading on these features so you understand what this code does. Then you should be in a better position to leverage the power of C++.
C++17 has a new attribute, [[nodiscard]].
Suppose, that I have a Result struct, which has this attribute:
struct [[nodiscard]] Result {
};
Now, if I call a function which returns Result, I got a warning if I don't check the returned Result:
Result someFunction();
int main() {
someFunction(); // warning here, as I don't check someFunction's return value
}
This program generates:
warning: ignoring return value of function declared with 'nodiscard'
attribute [-Wunused-result]
So far, so good. Now suppose, that I have a special function, for which I still want to return Result, but I don't want this warning generated, if the check is omitted:
Result someNonCriticalFunction();
int main() {
someNonCriticalFunction(); // I don't want to generate a warning here
}
It is because, someNonCriticalFunction() does something non-critical (for example, something like printf - I bet that no-one checks printf's return value all the time); most cases, I don't care if it fails. But I still want it to return Result, as in some rare cases, I do need its Result.
Is it possible to do this somehow?
Possible solutions which I don't like:
I would not like calling it as (void)someNonCriticalFunction(), because this function is called a lot of times, it is awkward
creating a wrapper around someNonCriticalFunction(), which calls (void)someNonCriticalFunction(): I don't want to have a differently named function just because of this
removing [[nodiscard]] from Result, and add it to every function which returns Result
Why not make use of std::ignore from the <tuple> header—that would make the discard explicit:
[[nodiscard]] int MyFunction() { return 42; }
int main()
{
std::ignore = MyFunction();
return 0;
}
Compiler explorer of this code snippet: https://godbolt.org/z/eGPsjajz8
CPP Reference for std::ignore: https://en.cppreference.com/w/cpp/utility/tuple/ignore
I recommend the option you ruled out:
"removing [[nodiscard]] from Result, and add it to every function which returns Result."
But since you don't seem happy with it, here's another solution, using bog-standard inheritance:
struct [[nodiscard]] Result {
};
struct DiscardableResult: public Result {
};
For the functions where you can discard the result, use DiscardableResult as return type:
Result func1();
DiscardableResult func2();
func1(); // will warn
func2(); // will not warn
They say that every problem in computer science can be solved by adding another layer of indirection:
template <bool nodiscard=true>
struct Result;
template <>
struct Result<false> {
// the actual implementation
};
template <>
struct [[nodiscard]] Result<true>
: Result<false>
{
using Result<false>::Result;
};
This is effectively making Result conditionally [[nodiscard]], which allows:
Result<true> someFunction();
Result<false> someNonCriticalFunction();
int main() {
someFunction(); // warning here
someNonCriticalFunction(); // no warning here
}
Although really, this is identical to:
removing [[nodiscard]] from Result, and add it to every function which returns Result
which gets my vote to begin with.
You can suppress the warning with another C++17 attribute, namely [[maybe_unused]]
[[nodiscard]] int MyFunction() { return 42; }
int main()
{
[[maybe_unused]] auto v = MyFunction();
return 0;
}
This way you also avoid the confusing dependency to std::tuple which comes with std::ignore, even CppCoreGuidelines is openly recommending to use std::ignore for ignoring [[nodiscard]] values:
Never cast to (void) to ignore a [[nodiscard]]return value. If you
deliberately want to discard such a result, first think hard about
whether that is really a good idea (there is usually a good reason the
author of the function or of the return type used [[nodiscard]] in the
first place). If you still think it's appropriate and your code
reviewer agrees, use std::ignore = to turn off the warning which is
simple, portable, and easy to grep.
Looking at C++ reference, officially std::ignore is only specified to be used in std::tie when unpacking tuples.
While the behavior of std::ignore outside of std::tie is not formally
specified, some code guides recommend using std::ignore to avoid
warnings from unused return values of [[nodiscard]] functions.
cast the result to a (void *).
int main()
{
(void *)someFunction(); //Warning will be gone.
}
This way you "used" your result as far as the compiler is concerned. Great for when you are using a library where nodiscard has been used and you really don't care to know the result.
I have the following code:
Foo a;
if (some_fairly_long_condition) {
a = complicated_expression_to_make_foo_1();
} else {
a = complicated_expression_to_make_foo_2();
}
I have two issues with this:
a is a const and should be declared so
the "empty" constructor, Foo() is called for no reason (maybe this is optimised away?)
One way to fix it is by using the ternary operator:
const Foo a = some_fairly_long_condition?
complicated_expression_to_make_foo_1():
complicated_expression_to_make_foo_2();
Is this good practice? How do you go about it?
To answer the second part of your question:
I usually put the initialization code into a lambda:
const Foo a = [&]()->Foo{
if (some_fairly_long_condition) {
return complicated_expression_to_make_foo_1();
} else {
return complicated_expression_to_make_foo_2();
}
}();
In most cases you should even be able to omit the trailing return type, so you can write
const Foo a = [&](){ ...
As far as the first part is concerned:
I'd say that greatly depends on how complex your initialization code is. If all three parts are really complicated expressions (and not just a function call each) then the solution with the ternary operator becomes an unreadable mess, while the lambda method (or a separate named function for that matter) allows you to break up those parts into the respective sub expressions.
If the problem is to avoid ternaty operator and your goal is to define the constant a, this code is an option:
Foo aux;
if (some_fairly_long_condition) {
aux = complicated_expression_to_make_foo_1();
} else {
aux = complicated_expression_to_make_foo_2();
}
const Foo a(aux);
It is a good solution, without any new feature ---as lambdas--- and including the code inline, as you want.
In C/C++, we have the __FUNCTION__ macro which is replaced with a string, holding the name of the current function. But what if I want the function's identifier? That is, not a string, but something I could use as a token to create other identifiers, e.g., if we have
#define MAGIC /* ... */
#define MORE_MAGIC MAGIC ## _bar
void foo() {
printf("%s\n",__FUNCTION__);
MORE_MAGIC();
}
void foo_bar() {
printf("%s\n",__FUNCTION__);
}
void baz() {
printf("%s\n",__FUNCTION__);
MORE_MAGIC();
}
void baz_bar() {
printf("%s\n",__FUNCTION__);
}
int main() {
foo();
}
should print
foo
foo_bar
baz
baz_bar
Notes:
I'm interested in preprocessing-time only.
I would rather not replace my function definitions with a preprocessor call - although I know that would probably work.
Unfortunately you can't. Because you can not unstringify a macro[1].
In other words, you can not remove quotes around the string that generated by __FUNCTION__ and contact it by _bar.
If it's compile-time you want, and for a simple case like your, it might be possible with preprocessor macros and the concatenation operator ##. Maybe something like
#define MORE_MAGIC(f) f##_bar
...
void foo_bar()
{
}
void foo()
{
MORE_MAGIC(foo)();
}
It's not possible to get the name foo automatically though, it has to be explicitly named in the macro "call".
So I have a series of global functions, say:
foo_f1(int a, int b, char *c);
foo_f2(int a);
foo_f3(char *a);
I want to make a C++ wrapper around these, something like:
MyFoo::f1(int a, int b, char* c);
MyFoo::f2(int a);
MyFoo::f3(char* a);
There's about 40 functions like this, 35 of them I just want to pass through to the global function, the other 5 I want to do something different with.
Ideally the implementation of MyFoo.cpp would be something like:
PASSTHRU( f1, (int a, int b, char *c) );
PASSTHRU( f2, (int a) );
MyFoo::f3(char *a)
{
//do my own thing here
}
But I'm having trouble figuring out an elegant way to make the above PASSTHRU macro.
What I really need is something like the mythical X getArgs() below:
MyFoo::f1(int a, int b, char *c)
{
X args = getArgs();
args++; //skip past implicit this..
::f1(args); //pass args to global function
}
But short of dropping into assembly I can't find a good implementation of getArgs().
You could use Boost.Preprocessor to let the following:
struct X {
PASSTHRU(foo, void, (int)(char))
};
... expand to:
struct X {
void foo ( int arg0 , char arg1 ) { return ::foo ( arg0 , arg1 ); }
};
... using these macros:
#define DO_MAKE_ARGS(r, data, i, type) \
BOOST_PP_COMMA_IF(i) type arg##i
#define PASSTHRU(name, ret, args) \
ret name ( \
BOOST_PP_SEQ_FOR_EACH_I(DO_MAKE_ARGS, _, args) \
) { \
return ::name ( \
BOOST_PP_ENUM_PARAMS(BOOST_PP_SEQ_SIZE(args), arg) \
); \
}
At 40-odd functions, you could type the wrappers out by hand in an hour. The compiler will check the correctness of the result. Assume an extra 2 minutes for each new function that needs wrapping, and an extra 1 minute for a change in signature.
As specified, and with no mention of frequent updates or changes, it doesn't sound like this problem requires a cunning solution.
So, my recommendation is to keep it simple: do it by hand. Copy prototypes into source file, then use keyboard macros (emacs/Visual Studio/vim) to fix things up, and/or multiple passes of search and replace, generating one set of definitions and one set of declarations. Cut declarations, paste into header. Fill in definitions for the non-passing-through functions. This won't win you any awards, but it'll be over soon enough.
No extra dependencies, no new build tools, works well with code browsing/tags/intellisense/etc., works well with any debugger, and no specialized syntax/modern features/templates/etc., so anybody can understand the result. (It's true that nobody will be impressed -- but it will be the good kind of unimpressed.)
Slightly different syntax but...
#include <boost/preprocessor.hpp>
#include <iostream>
void f1(int x, int y, char* z) { std::cout << "::f1(int,int,char*)\n"; }
#define GENERATE_ARG(z,n,unused) BOOST_PP_CAT(arg,n)
#define GET_ARGS(n) BOOST_PP_ENUM(n, GENERATE_ARG, ~)
#define GENERATE_PARAM(z,n,seq) BOOST_PP_SEQ_ELEM(n,seq) GENERATE_ARG(z,n,~)
#define GENERATE_PARAMS(seq) BOOST_PP_ENUM( BOOST_PP_SEQ_SIZE(seq), GENERATE_PARAM, seq )
#define PASSTHROUGH(Classname, Function, ArgTypeSeq) \
void Classname::Function( GENERATE_PARAMS(ArgTypeSeq) ) \
{ \
::Function( GET_ARGS( BOOST_PP_SEQ_SIZE(ArgTypeSeq) ) ); \
}
struct test
{
void f1(int,int,char*);
};
PASSTHROUGH(test,f1,(int)(int)(char*))
int main()
{
test().f1(5,5,0);
std::cin.get();
}
You could get something closer to yours if you use tuples, but you'd have to supply the arg count to the base function (you can't derive a size from a tuple). Sort of like so:
PASSTHROUGH(test,f1,3,(int,int,char*))
That about what you're looking for? I knew it could be done; took about a half hour to solve. You seem to expect that there's an implicit 'this' that has to be gotten rid of but I don't see why...so maybe I misunderstand the problem. At any rate, this will let you quickly make default "passthrough" member functions that defer to some global function. You'll need a DECPASSTHROUGH for the class declaration if you want to skip having to declare them all...or you could modify this to make inline functions.
Hint: Use BOOST_PP_STRINGIZE((XX)) to test the output of preprocessor metafunctions.
My initial thought, and this probably won't work or others would have stated this, is to put all your base functions together in a class as virtual. Then, write the functionality improvements into inherited classes and run with it. It's not a macro wrapper, but you could always call the global functions in the virtual classes.
With some assembly trickery, you could probably do exactly what you'd want, but you would lose portability more than likely. Interesting question and I want to hear other's answers as well.
You may want to use a namespace if you want to not deal with class stuff, like this. You could also use static member methods in a class, but I think that people don't like that anymore.
#ifndef __cplusplus
#define PASSTHRU(type, prefix, func, args) type prefix##_##func args
#else
#define PASSTHRU(type, prefix, func, args) type prefix::func args
#endif
Or
#ifndef __cplusplus
#define PASSTHRU(type, prefix, func, ...) type prefix##_##func(__VA_ARGS__)
...
Perfect forwarding relies on rvalue references. STL has a blog entry on it at Link and you would want to choose a compiler that supported the feature to take this approach. He's discussing Visual C++ 2010.