I'm new to C++ and very confused on how to approach this. In Javascript, I can do something like this to access an object dynamically very easily:
function someItem(prop) {
const item = {
prop1: 'hey',
prop2: 'hello'
};
return item[prop];
}
In C++, I'm assuming I have to use a Struct, but after that I'm stuck on how to access the struct member variables dynamically.
void SomeItem(Property Prop)
{
struct Item
{
Proper Prop1;
Proper Prop2;
};
// Item[Prop] ??
}
This could be terrible code but I'm very confused on how to approach this.
This is a simple example of how to create an instance of a struct and then access its members:
#include <iostream>
#include <string>
struct Item {
std::string prop1 = "hey";
std::string prop2 = "hello";
};
int main() {
Item myItem;
std::cout << myItem.prop1 << std::endl; // This prints "hey"
std::cout << myItem.prop2 << std::endl; // This prints "hello"
return 0;
}
As mentioned in the comments, it looks like you might want a map. A map has keys and values associated with them, as an example you could have a key "prop1" be associated with a value "hey":
#include <iostream>
#include <map>
#include <string>
int main() {
std::map<std::string, std::string> myMap;
myMap["prop1"] = "hey";
myMap["prop2"] = "hello";
std::cout << myMap["prop1"] << std::endl; // This print "hey"
std::cout << myMap["prop2"] << std::endl; // This print "hello"
return 0;
}
The first would be considered "normal" struct usage in C++ and the other is more applicable to cases where you have to look things up by keys
As mentioned in a comment, in C++ you would not define a custom structure for this, but rather use a std::unordered_map. I don't know Javascript, though if Property is an enum (it could be something else with small modifications) and return item[prop]; is supposed to return a string, then this might be close:
#include <string>
#include <unordered_map>
#include <iostream>
enum class Property { prop1,prop2};
std::string someItem(Property p){
const std::unordered_map<Property,std::string> item{
{Property::prop1,"hey"},
{Property::prop2,"hello"}
};
auto it = item.find(p);
if (it == item.end()) throw "unknown prop";
return it->second;
}
int main(){
std::cout << someItem(Property::prop1);
}
std::unordered_map does have a operator[] that you could use like so return item[p];, but it inserts an element into the map when none is found for the given key. This is not always desirable, and not possible when the map is const.
I want my function to return a string, but only strings which are a member of a specific list/set of strings. How can I go about doing this?
You do not want to return a string, you want to return a string that has an additional restriction (being part of some predefined set).
For that you'd need a new type:
class BusinessStringWrapper {
public:
BusinessStringWrapper(std::string arg): value{arg} {
if (/* arg is not ok */) {
throw;
}
}
// you can replace that with factory method
// can also return std::optional instead of throwing if the condition is not met
// that depends on your application
std::string value() const { return value; }
private:
const std::string value;
};
And in your application you'd operate on this type, accessing value if needed.
Hoe about using a std::set<std::string>?
#include <iostream>
#include <set>
#include <string>
std::string helper(const std::string & str,
const std::set<std::string> & lst)
{
return lst.find(str) == lst.end() ? "" : str;
}
int main()
{
std::set<std::string> lst = {"alpha", "beta", "gamma"};
std::cout << "return " << helper("alpha", lst) << "\n";
std::cout << "return " << helper("zeta", lst) << "\n";
return 0;
}
Output
return alpha
return
Of course, it really depends on what your definition of does not return is.
If it means an empty string, then use the above solution. Keep your life simple.
If it means an error and the program should terminate, you may #include <cassert> and just
assert(lst.find(str) != lst.end());
If it means an exception to handle, you may try throw and catch.
If it means returning a std::string if str is in a predefined list, but a void if it's not, then you may need some tricks as described in <type_traits>.
You can do this std::map<CardType, std::string> in the example below, or use std::map<int, std::string> to associate a string with any integer. For example mp[123]="abcd"
#include <iostream>
#include <string>
#include <map>
enum CardType {
SPADE,
HEART,
CLUBS,
DIAMD
};
std::map<CardType, std::string> mp{
{CardType::SPADE, "Spade"},
{CardType::HEART, "Heart"},
{CardType::CLUBS, "Clubs"},
{CardType::DIAMD, "Diamond"}
};
int main()
{
std::cout << mp[CardType::SPADE] << std::endl;
return 0;
}
I'm facing a weird issue : I can't reset (destruct and construct) properly an attribute containing a vector. It causes a segmentation fault when trying to access the vector.
Here is my code (witten in C++11). I think I simplified it the most possible to underscore the issue, but I might be wrong, sorry about that.
The goal would be to print two times two different (random) vectors. The first vector is working well, the second is completely failing for an unknown reason.
#include <iostream>
#include <ctime>
#include <cstdlib>
#include <vector>
class A
{
std::vector<int> n;
public :
A();
std::string toString() const;
};
A::A()
{
for (int i = 0; i < 10; i++)
n.push_back(std::rand()%10);
}
std::string A::toString() const
{
for (auto i : n)
std::cout << i << ' ';
std::cout << std::endl;
}
class B
{
A a;
public :
void resetA();
A getA() const;
};
void B::resetA()
{
a = A();
}
A B::getA() const
{
return a;
}
int main()
{
srand(time(NULL));
B b;
std::cout << b.getA().toString();
b.resetA();
std::cout << b.getA().toString();
return EXIT_SUCCESS;
}
For some reason, I would like to avoid pointers and dynamic allocation as far as possible. It would fit less with my UML conception.
Moreover, this code is working well when using simple int (no vectors).
Thank you.
Your toString() doesn't return anything, so your program has Undefined Behaviour (and, in practice, returns random garbage which is most certainly not a valid std::string object).
Perhaps you wanted to use a string stream instead?
#include <sstream>
// ...
std::string A::toString() const
{
std::ostringstream s;
for (auto i : n)
s << i << ' ';
s << '\n';
return s.str();
}
Live example.
Generally, it's a good idea to compile with as many warnings turned on as possible. This would certainly have been reported as a warning then. For this particular warning (no-void function not returning anything), I strongly suggest treating it as an error.
I have a large series of functions that all look very similar: they take the same arguement type and return strings.
std::string f1(T arg);
std::string f2(T arg);
std::string f3(T arg);
std::string f4(T arg);
.
.
.
In a loop, they are used according to one of the variables inside the struct T. Currently to do this, I just have a large switch/case block in my code.
Is there any better coding style for doing this? The large block of code looks very weird.
I wish c++ could be like python and do eval("f" + str(i) + "(arg))"
The block is something like this:
std::string out = "";
switch (arg.tag){
case 1:
out += f1(arg);
break;
case 2:
out += f2(arg);
break;
.
.
.
}
for about 2 dozen cases
With C++11 you can do this fairly easily with std::function and a map:
#include <map>
#include <functional>
#include <string>
#include <iostream>
std::string f1(int) { return "f1"; }
std::string f2(int) { return "f2"; }
std::map<int, std::function<std::string(int)> > funcs = {
{1,f1},
{2,f2}
};
int main() {
std::cout << funcs[1](100) << "\n";
}
Without C++11 you'll want to either use Boost instead of std::function or roll your own type instead. You could use plain old function pointers but that would rule out some handy things (like std::bind/boost::bind, functor objects, lambda functions. You could also define a type hierarchy with an interface that your functions implement for example the following works in C++03 except for the way the map is initialised:
#include <map>
#include <functional>
#include <string>
#include <iostream>
std::string f1(int) { return "f1"; }
std::string f2(int) { return "f2"; }
std::map<int, std::string(*)(int)> funcs = {
std::make_pair(1,f1),
std::make_pair(2,f2)
};
int main() {
std::cout << funcs[1](100) << "\n";
}
or this which lets you write any kind of functor object you like:
#include <map>
#include <string>
#include <iostream>
struct thing {
virtual std::string operator()(int) const = 0;
};
struct f1 : thing {
std::string operator()(int) const { return "f1"; }
};
struct f2 : thing {
std::string operator()(int) const { return "f2"; }
};
// Note the leak - these never get deleted:
std::map<int, thing*> funcs = {
std::make_pair(1,new f1),
std::make_pair(2,new f2)
};
int main() {
std::cout << (*funcs[1])(100) << "\n";
}
One way to emulate the Eval() is to have a map. The key of the map would be the names of the functions, and the values would be the pointers to the corresponding functions.
In this case you will be able to call the functions needed with the map's operator[] by their name. This will somehow emulate the eval("f" + str(i) + "(arg))" behavior, though it may still not be the best solution for you.
I'm currently working on some logging code that supposed to - among other things - print information about the calling function. This should be relatively easy, standard C++ has a type_info class. This contains the name of the typeid'd class/function/etc. but it's mangled. It's not very useful. I.e. typeid(std::vector<int>).name() returns St6vectorIiSaIiEE.
Is there a way to produce something useful from this? Like std::vector<int> for the above example. If it only works for non-template classes, that's fine too.
The solution should work for gcc, but it would be better if I could port it. It's for logging so it's not so important that it can't be turned off, but it should be helpful for debugging.
Given the attention this question / answer receives, and the valuable feedback from GManNickG, I have cleaned up the code a little bit. Two versions are given: one with C++11 features and another one with only C++98 features.
In file type.hpp
#ifndef TYPE_HPP
#define TYPE_HPP
#include <string>
#include <typeinfo>
std::string demangle(const char* name);
template <class T>
std::string type(const T& t) {
return demangle(typeid(t).name());
}
#endif
In file type.cpp (requires C++11)
#include "type.hpp"
#ifdef __GNUG__
#include <cstdlib>
#include <memory>
#include <cxxabi.h>
std::string demangle(const char* name) {
int status = -4; // some arbitrary value to eliminate the compiler warning
// enable c++11 by passing the flag -std=c++11 to g++
std::unique_ptr<char, void(*)(void*)> res {
abi::__cxa_demangle(name, NULL, NULL, &status),
std::free
};
return (status==0) ? res.get() : name ;
}
#else
// does nothing if not g++
std::string demangle(const char* name) {
return name;
}
#endif
Usage:
#include <iostream>
#include "type.hpp"
struct Base { virtual ~Base() {} };
struct Derived : public Base { };
int main() {
Base* ptr_base = new Derived(); // Please use smart pointers in YOUR code!
std::cout << "Type of ptr_base: " << type(ptr_base) << std::endl;
std::cout << "Type of pointee: " << type(*ptr_base) << std::endl;
delete ptr_base;
}
It prints:
Type of ptr_base: Base*
Type of pointee: Derived
Tested with g++ 4.7.2, g++ 4.9.0 20140302 (experimental), clang++ 3.4 (trunk 184647), clang 3.5 (trunk 202594) on Linux 64 bit and g++ 4.7.2 (Mingw32, Win32 XP SP2).
If you cannot use C++11 features, here is how it can be done in C++98, the file type.cpp is now:
#include "type.hpp"
#ifdef __GNUG__
#include <cstdlib>
#include <memory>
#include <cxxabi.h>
struct handle {
char* p;
handle(char* ptr) : p(ptr) { }
~handle() { std::free(p); }
};
std::string demangle(const char* name) {
int status = -4; // some arbitrary value to eliminate the compiler warning
handle result( abi::__cxa_demangle(name, NULL, NULL, &status) );
return (status==0) ? result.p : name ;
}
#else
// does nothing if not g++
std::string demangle(const char* name) {
return name;
}
#endif
(Update from Sep 8, 2013)
The accepted answer (as of Sep 7, 2013), when the call to abi::__cxa_demangle() is successful, returns a pointer to a local, stack allocated array... ouch!
Also note that if you provide a buffer, abi::__cxa_demangle() assumes it to be allocated on the heap. Allocating the buffer on the stack is a bug (from the gnu doc): "If output_buffer is not long enough, it is expanded using realloc." Calling realloc() on a pointer to the stack... ouch! (See also Igor Skochinsky's kind comment.)
You can easily verify both of these bugs: just reduce the buffer size in the accepted answer (as of Sep 7, 2013) from 1024 to something smaller, for example 16, and give it something with a name not longer than 15 (so realloc() is not called). Still, depending on your system and the compiler optimizations, the output will be: garbage / nothing / program crash.
To verify the second bug: set the buffer size to 1 and call it with something whose name is longer than 1 character. When you run it, the program almost assuredly crashes as it attempts to call realloc() with a pointer to the stack.
(The old answer from Dec 27, 2010)
Important changes made to KeithB's code: the buffer has to be either allocated by malloc or specified as NULL. Do NOT allocate it on the stack.
It's wise to check that status as well.
I failed to find HAVE_CXA_DEMANGLE. I check __GNUG__ although that does not guarantee that the code will even compile. Anyone has a better idea?
#include <cxxabi.h>
const string demangle(const char* name) {
int status = -4;
char* res = abi::__cxa_demangle(name, NULL, NULL, &status);
const char* const demangled_name = (status==0)?res:name;
string ret_val(demangled_name);
free(res);
return ret_val;
}
Boost core contains a demangler. Checkout core/demangle.hpp:
#include <boost/core/demangle.hpp>
#include <typeinfo>
#include <iostream>
template<class T> struct X
{
};
int main()
{
char const * name = typeid( X<int> ).name();
std::cout << name << std::endl; // prints 1XIiE
std::cout << boost::core::demangle( name ) << std::endl; // prints X<int>
}
It's basically just a wrapper for abi::__cxa_demangle, as has been suggested previously.
If all we want is the unmangled type name for the purpose of logging, we can actually do this without using std::type_info or even RTTI at all.
A slightly portable solution that works for the big 3 main compiler front-ends (gcc, clang, and msvc) would be to use a function template and extract the type name from the function name.
gcc and clang both offer __PRETTY_FUNCTION__ which is the name of a current function or function template with all type-argument in the string. Similarly MSVC has __FUNCSIG__ which is equivalent. Each of these are formatted a little differently, for example, for a call of void foo<int>, the compilers will output something different:
gcc is formatted void foo() [with T = int; ]
clang is formatted void foo() [T = int]
msvc is formatted void foo<int>()
Knowing this, it's just a matter of parsing out a prefix and suffix and wrapping this into a function in order to extract out the type name.
We can even use c++17's std::string_view and extended constexpr to get string names at compile-time, just by parsing the name of a template function. This could also be done in any earlier C++ version, but this will still require some form of string parsing.
For example:
#include <string_view>
template <typename T>
constexpr auto get_type_name() -> std::string_view
{
#if defined(__clang__)
constexpr auto prefix = std::string_view{"[T = "};
constexpr auto suffix = "]";
constexpr auto function = std::string_view{__PRETTY_FUNCTION__};
#elif defined(__GNUC__)
constexpr auto prefix = std::string_view{"with T = "};
constexpr auto suffix = "; ";
constexpr auto function = std::string_view{__PRETTY_FUNCTION__};
#elif defined(_MSC_VER)
constexpr auto prefix = std::string_view{"get_type_name<"};
constexpr auto suffix = ">(void)";
constexpr auto function = std::string_view{__FUNCSIG__};
#else
# error Unsupported compiler
#endif
const auto start = function.find(prefix) + prefix.size();
const auto end = function.find(suffix);
const auto size = end - start;
return function.substr(start, size);
}
With this, you can call get_type_name<T>() to get a std::string_view at compile-time indicating the unmangled type name.
For example:
std::cout << get_type_name<std::string>() << std::endl;
on GCC will output:
std::__cxx11::basic_string<char>
and on clang will output:
std::basic_string<char>
Live Example
A similar augmentation to this approach which avoids a prefix and suffix is to assume that the function name is the same for all types, and search for a sentinel type to parse out the offset to the sentinel from each end. This ensures that the string searching only happens once, and the offset is assumed to find the string name each time. For example, using double as a simple sentinel:
template <typename T>
constexpr auto full_function_name() -> std::string_view
{
#if defined(__clang__) || defined(__GNUC__)
return std::string_view{__PRETTY_FUNCTION__};
#elif defined(_MSC_VER)
return std::string_view{__FUNCSIG__};
#else
# error Unsupported compiler
#endif
}
// Outside of the template so its computed once
struct type_name_info {
static constexpr auto sentinel_function = full_function_name<double>();
static constexpr auto prefix_offset = sentinel_function.find("double");
static constexpr auto suffix_offset = sentinel_function.size() - prefix_offset - /* strlen("double") */ 6;
};
template <typename T>
constexpr auto get_type_name() -> std::string_view
{
constexpr auto function = full_function_name<T>();
const auto start = type_name_info::prefix_offset;
const auto end = function.size() - type_name_info::suffix_offset;
const auto size = end - start;
return function.substr(start, size);
}
Live Example
This isn't portable to all compilers, but can be modified for any compiler that offers a __FUNCSIG__/__PRETTY_FUNCTION__ equivalent; it just requires a bit of parsing.
note: This hasn't been fully tested, so there may be some bugs; but the primary idea is to parse any output that contains the name in totality -- which is often a side-effect of __func__-like outputs on compilers.
This is what we use. HAVE_CXA_DEMANGLE is only set if available (recent versions of GCC only).
#ifdef HAVE_CXA_DEMANGLE
const char* demangle(const char* name)
{
char buf[1024];
unsigned int size=1024;
int status;
char* res = abi::__cxa_demangle (name,
buf,
&size,
&status);
return res;
}
#else
const char* demangle(const char* name)
{
return name;
}
#endif
Here, take a look at type_strings.hpp it contains a function that does what you want.
If you just look for a demangling tool, which you e.g. could use to mangle stuff shown in a log file, take a look at c++filt, which comes with binutils. It can demangle C++ and Java symbol names.
It's implementation defined, so it's not something that's going to be portable. In MSVC++, name() is the undecorated name, and you have to look at raw_name() to get the decorated one.
Just a stab in the dark here, but under gcc, you might want to look at demangle.h
I also found a macro called __PRETTY_FUNCTION__, which does the trick. It gives a pretty function name (figures :)). This is what I needed.
I.e. it gives me the following:
virtual bool mutex::do_unlock()
But I don't think it works on other compilers.
The accepted solution [1] works mostly well.
I found at least one case (and I wouldn't call it a corner case) where it does not report what I expected... with references.
For those cases, I found another solution, posted at the bottom.
Problematic case (using type as defined in [1]):
int i = 1;
cout << "Type of " << "i" << " is " << type(i) << endl;
int & ri = i;
cout << "Type of " << "ri" << " is " << type(ri) << endl;
produces
Type of i is int
Type of ri is int
Solution (using type_name<decltype(obj)>(), see code below):
cout << "Type of " << "i" << " is " << type_name<decltype(i)>() << endl;
cout << "Type of " << "ri" << " is " << type_name<decltype(ri)>() << endl;
produces
Type of i is int
Type of ri is int&
as desired (at least by me)
Code
.
It has to be in an included header, not in a separately compiled source, due to specialization issues. See undefined reference to template function for instance.
#ifndef _MSC_VER
# include <cxxabi.h>
#endif
#include <memory>
#include <string>
#include <cstdlib>
template <class T>
std::string
type_name()
{
typedef typename std::remove_reference<T>::type TR;
std::unique_ptr<char, void(*)(void*)> own
(
#ifndef _MSC_VER
abi::__cxa_demangle(typeid(TR).name(), nullptr,
nullptr, nullptr),
#else
nullptr,
#endif
std::free
);
std::string r = own != nullptr ? own.get() : typeid(TR).name();
if (std::is_const<TR>::value)
r += " const";
if (std::is_volatile<TR>::value)
r += " volatile";
if (std::is_lvalue_reference<T>::value)
r += "&";
else if (std::is_rvalue_reference<T>::value)
r += "&&";
return r;
}
Not a complete solution, but you may want to look at what some of the standard (or widely supported) macro's define. It's common in logging code to see the use of the macros:
__FUNCTION__
__FILE__
__LINE__
e.g.:
log(__FILE__, __LINE__, __FUNCTION__, mymessage);
A slight variation on Ali's solution. If you want the code to still be very similar to
typeid(bla).name(),
writing this instead
Typeid(bla).name() (differing only in capital first letter)
then you may be interested in this:
In file type.hpp
#ifndef TYPE_HPP
#define TYPE_HPP
#include <string>
#include <typeinfo>
std::string demangle(const char* name);
/*
template <class T>
std::string type(const T& t) {
return demangle(typeid(t).name());
}
*/
class Typeid {
public:
template <class T>
Typeid(const T& t) : typ(typeid(t)) {}
std::string name() { return demangle(typ.name()); }
private:
const std::type_info& typ;
};
#endif
type.cpp stays same as in Ali's solution
Following Ali's solution, here is the C++11 templated alternative which worked best for my usage.
// type.h
#include <cstdlib>
#include <memory>
#include <cxxabi.h>
template <typename T>
std::string demangle() {
int status = -4;
std::unique_ptr<char, void (*)(void*)> res{
abi::__cxa_demangle(typeid(T).name(), NULL, NULL, &status), std::free};
return (status == 0) ? res.get() : typeid(T).name();
}
Usage:
// main.cpp
#include <iostream>
namespace test {
struct SomeStruct {};
}
int main()
{
std::cout << demangle<double>() << std::endl;
std::cout << demangle<const int&>() << std::endl;
std::cout << demangle<test::SomeStruct>() << std::endl;
return 0;
}
Will print:
double
int
test::SomeStruct
Take a look at __cxa_demangle which you can find at cxxabi.h.
// KeithB's solution is good, but has one serious flaw in that unless buf is static
// it'll get trashed from the stack before it is returned in res - and will point who-knows-where
// Here's that problem fixed, but the code is still non-re-entrant and not thread-safe.
// Anyone care to improve it?
#include <cxxabi.h>
// todo: javadoc this properly
const char* demangle(const char* name)
{
static char buf[1024];
size_t size = sizeof(buf);
int status;
// todo:
char* res = abi::__cxa_demangle (name,
buf,
&size,
&status);
buf[sizeof(buf) - 1] = 0; // I'd hope __cxa_demangle does this when the name is huge, but just in case.
return res;
}
I've always wanted to use type_info, but I'm sure that the result of the name() member function is non-standard and won't necessarily return anything that can be converted to a meaningful result.
If you are sticking to one compiler, there maybe a compiler specific function that will do what you want. Check the documentation.
boost::typeindex provides something helpful.
#include <boost/type_index.hpp>
#include <iostream>
#include <vector>
class Widget {};
int main() {
using boost::typeindex::type_id_with_cvr;
const std::vector<Widget> vw;
std::cout << type_id_with_cvr<decltype(vw)>().pretty_name() << std::endl;
std::cout << type_id_with_cvr<decltype(vw[0])>().pretty_name() << std::endl;
return 0;
}
The output is
std::vector<Widget, std::allocator<Widget> > const
Widget const&
What is worthy of notice is that type_id_with_cvr preserves reference and c/v qualifiers, while typeid doesn't. See the following example:
#include <iostream>
#include <boost/type_index.hpp>
#include <typeindex>
#include <vector>
#include <typeinfo>
class Widget {};
template <typename T>
void f(const T ¶m) {
std::cout << typeid(param).name() << std::endl;
std::cout
<< boost::typeindex::type_id_with_cvr<decltype(param)>().pretty_name()
<< std::endl;
}
int main() {
const std::vector<Widget> vw(1);
f(&vw[0]);
return 0;
}
The output is
PK6Widget
Widget const* const&
Here, typeid produces PK6Widget, which means Pointer to Konst Widget. The number '6' is the length of the name 'Widget'. This is not the correct type of param, in which the reference and const qualifier are dropped.
The type_id_with_cvr actually uses the demangling functions in boost::core, as has been mentioned in this answer. To preserve the cv qualifiers or reference, it just defines an empty template named cvr_saver and then passes cvr_saver<type> to typeid.
Effective Modern C++ Item 4 talks about this.