Related
Consider a class whose default constructor takes in the file path as a parameter.
class Test
{
public:
Test(const std::string& filepath);
...
...
};
Now I wish to create and initialize an array of Test objects using unique pointers in VS2017.
int main()
{
std::unique_ptr<Test[]> m_Tests;
int testCount = 2;
std::string path1, path2;
m_Tests = std::make_unique<Test[]>(testCount); // This line gives a compilation error
m_Tests[0] = std::make_unique<Test>(path1);
m_Tests[1] = std::make_unique<Test>(path2);
}
How can I make this work?
g++ 9.2.0 tells me that you lack default constructor, i.e. one without parameters. Adding such constructor works fine. If it's not what you want, you can create array of unique_ptr's, so std::unique_ptr<std::unique_ptr<Test>[]> and after that initialize each element by hand, something similar to this:
#include <memory>
#include <algorithm>
#include <iostream>
struct Test {
std::string str_;
Test(std::string const& str) : str_(str) { }
void print() { std::cout << str_ << '\n'; }
};
int main()
{
std::unique_ptr<std::unique_ptr<Test>[]> m_Tests;
int testCount = 2;
std::string path1{"a"}, path2{"b"};
m_Tests = std::make_unique<std::unique_ptr<Test>[]>(testCount);
std::array<std::string, 2> paths{path1, path2};
std::transform(paths.begin(), paths.end(), &m_Tests[0],
[](auto const& p) { return std::make_unique<Test>(p); });
for (int i = 0 ; i < testCount ; ++i) {
m_Tests[i]->print();
}
}
There is no overload of std::make_unique that does this, so you would need to use new directly:
m_Tests.reset(new Test[testCount]{path1, path2});
This will however only compile if testCount is a constant expression, so you need to change the definition int testCount = 2; to const int or constexpr int.
If testCount is not a constant expression, there needs to be a default constructor defined for the case that testCount is smaller than 2 at runtime.
So, really, you probably want to ignore testCount and just let the array size be deduced:
m_Tests.reset(new Test[]{path1, path2});
It would be much easier if you just used std::vector:
std::vector<Test> m_Tests;
//...
m_Tests.emplace_back(path1);
m_Tests.emplace_back(path2);
How about you use std::array and can you get rid of testCount (or use it as constexp) then the code can be like below.
class Test
{
public:
Test(const std::string& filepath){}
};
int main()
{
constexpr int testCount = 2;
std::unique_ptr<std::array<Test, testCount>> m_Tests;
std::string path1, path2;
m_Tests = std::make_unique<std::array<Test, testCount>>(std::array<Test, testCount>{path1,path2});
}
I wrote the code like the below, I don't know why the member variable flag did't equal 1 after sort process? Can someone give me suggestion.
Code:
class Func{
public:
int flag;
Func()
{
flag = 0;
}
bool operator()(int &l, int &r)
{
if(l==r)
{
flag = 1;
}
return l > r;
}
};
int _tmain(int argc, _TCHAR* argv[])
{
vector<int> a;
int b[] = {11,8,7,6,4,3,4,1};
for(int i = 0; i <sizeof(b)/sizeof(b[0]); i++)
{
a.push_back(b[i]);
}
Func FuncObj = Func();
std::sort(begin(a), end(a), FuncObj);
return 0;
}
According to reliable documentation, std::sort accepts the function object by value. This means the Func being used by std::sort is a copy of FuncObj. This means that when 4 is compared with 4, the copy's flag variable is set to 1, and FuncObj is unchanged. The copy is destroyed when std::sort returns, so this flag is lost.
The simplest solution define int flag as static int flag to that all instances of Func share the same Flag.
If this is not an option, different Funcs must have their own flag, my inclination was to go with a std::shared_ptr. Each default constructed Func would have had it's own flag and shared this flag with any copies made.
However Benjamin Lindley reminds of std::reference_wrapper and this provides a much more convenient solution for the problem. The wrapper is passed by value and copied rather then the Func it references, allowing the source Func to be modified inside std::sort.
#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
class Func{
public:
int flag;
Func(): flag(0)
{
}
bool operator()(int &l, int &r)
{
if(l==r)
{
flag = 1;
}
return l > r;
}
};
int main()
{
std::vector<int> a = {11,8,7,6,4,3,4,1};
Func FuncObj = Func();
std::sort(begin(a), end(a), std::reference_wrapper<Func>(FuncObj));
std::cout << FuncObj.flag;
return 0;
}
I came across this strange code snippet which compiles fine:
class Car
{
public:
int speed;
};
int main()
{
int Car::*pSpeed = &Car::speed;
return 0;
}
Why does C++ have this pointer to a non-static data member of a class? What is the use of this strange pointer in real code?
It's a "pointer to member" - the following code illustrates its use:
#include <iostream>
using namespace std;
class Car
{
public:
int speed;
};
int main()
{
int Car::*pSpeed = &Car::speed;
Car c1;
c1.speed = 1; // direct access
cout << "speed is " << c1.speed << endl;
c1.*pSpeed = 2; // access via pointer to member
cout << "speed is " << c1.speed << endl;
return 0;
}
As to why you would want to do that, well it gives you another level of indirection that can solve some tricky problems. But to be honest, I've never had to use them in my own code.
Edit: I can't think off-hand of a convincing use for pointers to member data. Pointer to member functions can be used in pluggable architectures, but once again producing an example in a small space defeats me. The following is my best (untested) try - an Apply function that would do some pre &post processing before applying a user-selected member function to an object:
void Apply( SomeClass * c, void (SomeClass::*func)() ) {
// do hefty pre-call processing
(c->*func)(); // call user specified function
// do hefty post-call processing
}
The parentheses around c->*func are necessary because the ->* operator has lower precedence than the function call operator.
This is the simplest example I can think of that conveys the rare cases where this feature is pertinent:
#include <iostream>
class bowl {
public:
int apples;
int oranges;
};
int count_fruit(bowl * begin, bowl * end, int bowl::*fruit)
{
int count = 0;
for (bowl * iterator = begin; iterator != end; ++ iterator)
count += iterator->*fruit;
return count;
}
int main()
{
bowl bowls[2] = {
{ 1, 2 },
{ 3, 5 }
};
std::cout << "I have " << count_fruit(bowls, bowls + 2, & bowl::apples) << " apples\n";
std::cout << "I have " << count_fruit(bowls, bowls + 2, & bowl::oranges) << " oranges\n";
return 0;
}
The thing to note here is the pointer passed in to count_fruit. This saves you having to write separate count_apples and count_oranges functions.
Another application are intrusive lists. The element type can tell the list what its next/prev pointers are. So the list does not use hard-coded names but can still use existing pointers:
// say this is some existing structure. And we want to use
// a list. We can tell it that the next pointer
// is apple::next.
struct apple {
int data;
apple * next;
};
// simple example of a minimal intrusive list. Could specify the
// member pointer as template argument too, if we wanted:
// template<typename E, E *E::*next_ptr>
template<typename E>
struct List {
List(E *E::*next_ptr):head(0), next_ptr(next_ptr) { }
void add(E &e) {
// access its next pointer by the member pointer
e.*next_ptr = head;
head = &e;
}
E * head;
E *E::*next_ptr;
};
int main() {
List<apple> lst(&apple::next);
apple a;
lst.add(a);
}
Here's a real-world example I am working on right now, from signal processing / control systems:
Suppose you have some structure that represents the data you are collecting:
struct Sample {
time_t time;
double value1;
double value2;
double value3;
};
Now suppose that you stuff them into a vector:
std::vector<Sample> samples;
... fill the vector ...
Now suppose that you want to calculate some function (say the mean) of one of the variables over a range of samples, and you want to factor this mean calculation into a function. The pointer-to-member makes it easy:
double Mean(std::vector<Sample>::const_iterator begin,
std::vector<Sample>::const_iterator end,
double Sample::* var)
{
float mean = 0;
int samples = 0;
for(; begin != end; begin++) {
const Sample& s = *begin;
mean += s.*var;
samples++;
}
mean /= samples;
return mean;
}
...
double mean = Mean(samples.begin(), samples.end(), &Sample::value2);
Note Edited 2016/08/05 for a more concise template-function approach
And, of course, you can template it to compute a mean for any forward-iterator and any value type that supports addition with itself and division by size_t:
template<typename Titer, typename S>
S mean(Titer begin, const Titer& end, S std::iterator_traits<Titer>::value_type::* var) {
using T = typename std::iterator_traits<Titer>::value_type;
S sum = 0;
size_t samples = 0;
for( ; begin != end ; ++begin ) {
const T& s = *begin;
sum += s.*var;
samples++;
}
return sum / samples;
}
struct Sample {
double x;
}
std::vector<Sample> samples { {1.0}, {2.0}, {3.0} };
double m = mean(samples.begin(), samples.end(), &Sample::x);
EDIT - The above code has performance implications
You should note, as I soon discovered, that the code above has some serious performance implications. The summary is that if you're calculating a summary statistic on a time series, or calculating an FFT etc, then you should store the values for each variable contiguously in memory. Otherwise, iterating over the series will cause a cache miss for every value retrieved.
Consider the performance of this code:
struct Sample {
float w, x, y, z;
};
std::vector<Sample> series = ...;
float sum = 0;
int samples = 0;
for(auto it = series.begin(); it != series.end(); it++) {
sum += *it.x;
samples++;
}
float mean = sum / samples;
On many architectures, one instance of Sample will fill a cache line. So on each iteration of the loop, one sample will be pulled from memory into the cache. 4 bytes from the cache line will be used and the rest thrown away, and the next iteration will result in another cache miss, memory access and so on.
Much better to do this:
struct Samples {
std::vector<float> w, x, y, z;
};
Samples series = ...;
float sum = 0;
float samples = 0;
for(auto it = series.x.begin(); it != series.x.end(); it++) {
sum += *it;
samples++;
}
float mean = sum / samples;
Now when the first x value is loaded from memory, the next three will also be loaded into the cache (supposing suitable alignment), meaning you don't need any values loaded for the next three iterations.
The above algorithm can be improved somewhat further through the use of SIMD instructions on eg SSE2 architectures. However, these work much better if the values are all contiguous in memory and you can use a single instruction to load four samples together (more in later SSE versions).
YMMV - design your data structures to suit your algorithm.
You can later access this member, on any instance:
int main()
{
int Car::*pSpeed = &Car::speed;
Car myCar;
Car yourCar;
int mySpeed = myCar.*pSpeed;
int yourSpeed = yourCar.*pSpeed;
assert(mySpeed > yourSpeed); // ;-)
return 0;
}
Note that you do need an instance to call it on, so it does not work like a delegate.
It is used rarely, I've needed it maybe once or twice in all my years.
Normally using an interface (i.e. a pure base class in C++) is the better design choice.
IBM has some more documentation on how to use this. Briefly, you're using the pointer as an offset into the class. You can't use these pointers apart from the class they refer to, so:
int Car::*pSpeed = &Car::speed;
Car mycar;
mycar.*pSpeed = 65;
It seems a little obscure, but one possible application is if you're trying to write code for deserializing generic data into many different object types, and your code needs to handle object types that it knows absolutely nothing about (for example, your code is in a library, and the objects into which you deserialize were created by a user of your library). The member pointers give you a generic, semi-legible way of referring to the individual data member offsets, without having to resort to typeless void * tricks the way you might for C structs.
It makes it possible to bind member variables and functions in the uniform manner. The following is example with your Car class. More common usage would be binding std::pair::first and ::second when using in STL algorithms and Boost on a map.
#include <list>
#include <algorithm>
#include <iostream>
#include <iterator>
#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
class Car {
public:
Car(int s): speed(s) {}
void drive() {
std::cout << "Driving at " << speed << " km/h" << std::endl;
}
int speed;
};
int main() {
using namespace std;
using namespace boost::lambda;
list<Car> l;
l.push_back(Car(10));
l.push_back(Car(140));
l.push_back(Car(130));
l.push_back(Car(60));
// Speeding cars
list<Car> s;
// Binding a value to a member variable.
// Find all cars with speed over 60 km/h.
remove_copy_if(l.begin(), l.end(),
back_inserter(s),
bind(&Car::speed, _1) <= 60);
// Binding a value to a member function.
// Call a function on each car.
for_each(s.begin(), s.end(), bind(&Car::drive, _1));
return 0;
}
You can use an array of pointer to (homogeneous) member data to enable a dual, named-member (i.e. x.data) and array-subscript (i.e. x[idx]) interface.
#include <cassert>
#include <cstddef>
struct vector3 {
float x;
float y;
float z;
float& operator[](std::size_t idx) {
static float vector3::*component[3] = {
&vector3::x, &vector3::y, &vector3::z
};
return this->*component[idx];
}
};
int main()
{
vector3 v = { 0.0f, 1.0f, 2.0f };
assert(&v[0] == &v.x);
assert(&v[1] == &v.y);
assert(&v[2] == &v.z);
for (std::size_t i = 0; i < 3; ++i) {
v[i] += 1.0f;
}
assert(v.x == 1.0f);
assert(v.y == 2.0f);
assert(v.z == 3.0f);
return 0;
}
One way I've used it is if I have two implementations of how to do something in a class and I want to choose one at run-time without having to continually go through an if statement i.e.
class Algorithm
{
public:
Algorithm() : m_impFn( &Algorithm::implementationA ) {}
void frequentlyCalled()
{
// Avoid if ( using A ) else if ( using B ) type of thing
(this->*m_impFn)();
}
private:
void implementationA() { /*...*/ }
void implementationB() { /*...*/ }
typedef void ( Algorithm::*IMP_FN ) ();
IMP_FN m_impFn;
};
Obviously this is only practically useful if you feel the code is being hammered enough that the if statement is slowing things done eg. deep in the guts of some intensive algorithm somewhere. I still think it's more elegant than the if statement even in situations where it has no practical use but that's just my opnion.
Pointers to classes are not real pointers; a class is a logical construct and has no physical existence in memory, however, when you construct a pointer to a member of a class it gives an offset into an object of the member's class where the member can be found; This gives an important conclusion: Since static members are not associated with any object so a pointer to a member CANNOT point to a static member(data or functions) whatsoever
Consider the following:
class x {
public:
int val;
x(int i) { val = i;}
int get_val() { return val; }
int d_val(int i) {return i+i; }
};
int main() {
int (x::* data) = &x::val; //pointer to data member
int (x::* func)(int) = &x::d_val; //pointer to function member
x ob1(1), ob2(2);
cout <<ob1.*data;
cout <<ob2.*data;
cout <<(ob1.*func)(ob1.*data);
cout <<(ob2.*func)(ob2.*data);
return 0;
}
Source: The Complete Reference C++ - Herbert Schildt 4th Edition
Here is an example where pointer to data members could be useful:
#include <iostream>
#include <list>
#include <string>
template <typename Container, typename T, typename DataPtr>
typename Container::value_type searchByDataMember (const Container& container, const T& t, DataPtr ptr) {
for (const typename Container::value_type& x : container) {
if (x->*ptr == t)
return x;
}
return typename Container::value_type{};
}
struct Object {
int ID, value;
std::string name;
Object (int i, int v, const std::string& n) : ID(i), value(v), name(n) {}
};
std::list<Object*> objects { new Object(5,6,"Sam"), new Object(11,7,"Mark"), new Object(9,12,"Rob"),
new Object(2,11,"Tom"), new Object(15,16,"John") };
int main() {
const Object* object = searchByDataMember (objects, 11, &Object::value);
std::cout << object->name << '\n'; // Tom
}
Suppose you have a structure. Inside of that structure are
* some sort of name
* two variables of the same type but with different meaning
struct foo {
std::string a;
std::string b;
};
Okay, now let's say you have a bunch of foos in a container:
// key: some sort of name, value: a foo instance
std::map<std::string, foo> container;
Okay, now suppose you load the data from separate sources, but the data is presented in the same fashion (eg, you need the same parsing method).
You could do something like this:
void readDataFromText(std::istream & input, std::map<std::string, foo> & container, std::string foo::*storage) {
std::string line, name, value;
// while lines are successfully retrieved
while (std::getline(input, line)) {
std::stringstream linestr(line);
if ( line.empty() ) {
continue;
}
// retrieve name and value
linestr >> name >> value;
// store value into correct storage, whichever one is correct
container[name].*storage = value;
}
}
std::map<std::string, foo> readValues() {
std::map<std::string, foo> foos;
std::ifstream a("input-a");
readDataFromText(a, foos, &foo::a);
std::ifstream b("input-b");
readDataFromText(b, foos, &foo::b);
return foos;
}
At this point, calling readValues() will return a container with a unison of "input-a" and "input-b"; all keys will be present, and foos with have either a or b or both.
Just to add some use cases for #anon's & #Oktalist's answer, here's a great reading material about pointer-to-member-function and pointer-to-member-data.
https://www.dre.vanderbilt.edu/~schmidt/PDF/C++-ptmf4.pdf
with pointer to member, we can write generic code like this
template<typename T, typename U>
struct alpha{
T U::*p_some_member;
};
struct beta{
int foo;
};
int main()
{
beta b{};
alpha<int, beta> a{&beta::foo};
b.*(a.p_some_member) = 4;
return 0;
}
I love the * and & operators:
struct X
{
int a {0};
int *ptr {NULL};
int &fa() { return a; }
int *&fptr() { return ptr; }
};
int main(void)
{
X x;
int X::*p1 = &X::a; // pointer-to-member 'int X::a'. Type of p1 = 'int X::*'
x.*p1 = 10;
int *X::*p2 = &X::ptr; // pointer-to-member-pointer 'int *X::ptr'. Type of p2 = 'int *X::*'
x.*p2 = nullptr;
X *xx;
xx->*p2 = nullptr;
int& (X::*p3)() = X::fa; // pointer-to-member-function 'X::fa'. Type of p3 = 'int &(X::*)()'
(x.*p3)() = 20;
(xx->*p3)() = 30;
int *&(X::*p4)() = X::fptr; // pointer-to-member-function 'X::fptr'. Type of p4 = 'int *&(X::*)()'
(x.*p4)() = nullptr;
(xx->*p4)() = nullptr;
}
Indeed all is true as long as the members are public, or static
I think you'd only want to do this if the member data was pretty large (e.g., an object of another pretty hefty class), and you have some external routine which only works on references to objects of that class. You don't want to copy the member object, so this lets you pass it around.
A realworld example of a pointer-to-member could be a more narrow aliasing constructor for std::shared_ptr:
template <typename T>
template <typename U>
shared_ptr<T>::shared_ptr(const shared_ptr<U>, T U::*member);
What that constructor would be good for
assume you have a struct foo:
struct foo {
int ival;
float fval;
};
If you have given a shared_ptr to a foo, you could then retrieve shared_ptr's to its members ival or fval using that constructor:
auto foo_shared = std::make_shared<foo>();
auto ival_shared = std::shared_ptr<int>(foo_shared, &foo::ival);
This would be useful if want to pass the pointer foo_shared->ival to some function which expects a shared_ptr
https://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr
Pointer to members are C++'s type safe equivalent for C's offsetof(), which is defined in stddef.h: Both return the information, where a certain field is located within a class or struct. While offsetof() may be used with certain simple enough classes also in C++, it fails miserably for the general case, especially with virtual base classes. So pointer to members were added to the standard. They also provide easier syntax to reference an actual field:
struct C { int a; int b; } c;
int C::* intptr = &C::a; // or &C::b, depending on the field wanted
c.*intptr += 1;
is much easier than:
struct C { int a; int b; } c;
int intoffset = offsetof(struct C, a);
* (int *) (((char *) (void *) &c) + intoffset) += 1;
As to why one wants to use offsetof() (or pointer to members), there are good answers elsewhere on stackoverflow. One example is here: How does the C offsetof macro work?
Right now I have a switch statement that takes in an input and choose one of the following actions:
Option 1
for(; i < queue_size; ++i)
{
prepared->setString(i+1, queue.at(i).model);
}
Option 2
for(; i < queue_size; ++i)
{
prepared->setString(i+1, queue.at(i).manufacturer);
}
Option 3
for(; i < queue_size; ++i)
{
prepared->setString(i+1, queue.at(i).name);
}
In PHP, you would be able to do the same doing something like this:
$queue[i][$member];
$member could then be set to "name", "manufacturer", etc.
Is there any way to do something similar or more robust in C++?
Thanks in advance for any help/suggestions!
Using C++11 std::function or boost::function if you don't have C++11:
std::map<YourSwitchType, std::function<void(void)> functionMap;
then define functions such as
void manufacturString() {
for(; i < queue_size; ++i) {
prepared->setString(i+1, queue.at(i).manufacturer);
}
}
for each case, and populate the map with these.
functionMap[someSwitchValue] = std::bind(&ThisType::manufactureString, this);
Then you can just call them:
functionMap[someSwitchValue]();
One advantage of this approach is that it doesn't limit you to member functions. You can put non-member functions, functors, static member and non-member functions in the same map. The only limitation is that after binding, they return void and take no arguments (that is specific to this example).
You could do this with a map from your property names to pointer-to-member. But it's a bit of work (you need to create that mapping yourself), and the syntax gets a bit hairy. (And all the members you want to address this way must be of the same type.)
Demo:
#include <iostream>
#include <map>
struct Foo {
std::string name;
std::string address;
};
typedef std::string Foo::* FooMemPtr;
typedef std::map<std::string, FooMemPtr> propmap;
int main()
{
propmap props;
props["name"] = &Foo::name;
props["address"] = &Foo::address;
/* ... */
Foo myfoo;
myfoo.*(props["name"]) = "myname";
myfoo.*(props["address"]) = "myaddress";
std::cout << myfoo.*(props["address"]) << std::endl;
std::cout << myfoo.*(props["name"]) << std::endl;
}
If you can use enums instead of strings, then you can access name, manufacturer, etc. indexed off of the enum values. It depends on how dynamic you need to be.
Use the STL map to perform this. It works as you would do in PHP.
One option is to pass an extractor functor to the function:
#include <string>
#include <vector>
#include <boost/bind.hpp>
struct Some {
std::string model, manufacturer, name;
};
struct Prepared {
void setString(size_t, std::string const&);
};
template<class Extractor>
void foo(Extractor extract) {
Prepared* prepared = 0;
std::vector<Some> queue;
size_t i, queue_size = queue.size();
for(; i < queue_size; ++i) {
prepared->setString(i+1, extract(queue.at(i)));
}
}
int main() {
// C++03
foo(boost::bind(&Some::model, _1));
foo(boost::bind(&Some::manufacturer, _1));
foo(boost::bind(&Some::name, _1));
// C++11
foo([](Some const& some){ return some.model; });
foo([](Some const& some){ return some.manufacturer; });
foo([](Some const& some){ return some.name; });
}
You can do this type of thing using member variable points (there are also member function pointers). The boost bind functions are more generic, but this is ultimately what they are doing underneath (at least in this scenario).
#include <iostream>
#include <string>
struct item
{
std::string a, b;
};
//the really ugly syntax for a pointer-to-member-variable
typedef std::string item::*mem_str;
//show each member variable from the list
void show_test( item * objs, size_t len, mem_str var )
{
for( size_t i=0; i < len; ++i )
std::cout << objs[i].*var << std::endl;
}
int main()
{
//create some test data
item test[2];
test[0].a = "A-Zero";
test[0].b = "B-Zero";
test[1].a = "A-One";
test[1].b = "B-One";
//show variables
show_test( test, 2, &item::a );
show_test( test, 2, &item::b );
return 0;
}
I have the following class in C++:
class a {
const int b[2];
// other stuff follows
// and here's the constructor
a(void);
}
The question is, how do I initialize b in the initialization list, given that I can't initialize it inside the body of the function of the constructor, because b is const?
This doesn't work:
a::a(void) :
b([2,3])
{
// other initialization stuff
}
Edit: The case in point is when I can have different values for b for different instances, but the values are known to be constant for the lifetime of the instance.
With C++11 the answer to this question has now changed and you can in fact do:
struct a {
const int b[2];
// other bits follow
// and here's the constructor
a();
};
a::a() :
b{2,3}
{
// other constructor work
}
int main() {
a a;
}
Like the others said, ISO C++ doesn't support that. But you can workaround it. Just use std::vector instead.
int* a = new int[N];
// fill a
class C {
const std::vector<int> v;
public:
C():v(a, a+N) {}
};
It is not possible in the current standard. I believe you'll be able to do this in C++0x using initializer lists (see A Brief Look at C++0x, by Bjarne Stroustrup, for more information about initializer lists and other nice C++0x features).
std::vector uses the heap. Geez, what a waste that would be just for the sake of a const sanity-check. The point of std::vector is dynamic growth at run-time, not any old syntax checking that should be done at compile-time. If you're not going to grow then create a class to wrap a normal array.
#include <stdio.h>
template <class Type, size_t MaxLength>
class ConstFixedSizeArrayFiller {
private:
size_t length;
public:
ConstFixedSizeArrayFiller() : length(0) {
}
virtual ~ConstFixedSizeArrayFiller() {
}
virtual void Fill(Type *array) = 0;
protected:
void add_element(Type *array, const Type & element)
{
if(length >= MaxLength) {
// todo: throw more appropriate out-of-bounds exception
throw 0;
}
array[length] = element;
length++;
}
};
template <class Type, size_t Length>
class ConstFixedSizeArray {
private:
Type array[Length];
public:
explicit ConstFixedSizeArray(
ConstFixedSizeArrayFiller<Type, Length> & filler
) {
filler.Fill(array);
}
const Type *Array() const {
return array;
}
size_t ArrayLength() const {
return Length;
}
};
class a {
private:
class b_filler : public ConstFixedSizeArrayFiller<int, 2> {
public:
virtual ~b_filler() {
}
virtual void Fill(int *array) {
add_element(array, 87);
add_element(array, 96);
}
};
const ConstFixedSizeArray<int, 2> b;
public:
a(void) : b(b_filler()) {
}
void print_items() {
size_t i;
for(i = 0; i < b.ArrayLength(); i++)
{
printf("%d\n", b.Array()[i]);
}
}
};
int main()
{
a x;
x.print_items();
return 0;
}
ConstFixedSizeArrayFiller and ConstFixedSizeArray are reusable.
The first allows run-time bounds checking while initializing the array (same as a vector might), which can later become const after this initialization.
The second allows the array to be allocated inside another object, which could be on the heap or simply the stack if that's where the object is. There's no waste of time allocating from the heap. It also performs compile-time const checking on the array.
b_filler is a tiny private class to provide the initialization values. The size of the array is checked at compile-time with the template arguments, so there's no chance of going out of bounds.
I'm sure there are more exotic ways to modify this. This is an initial stab. I think you can pretty much make up for any of the compiler's shortcoming with classes.
ISO standard C++ doesn't let you do this. If it did, the syntax would probably be:
a::a(void) :
b({2,3})
{
// other initialization stuff
}
Or something along those lines. From your question it actually sounds like what you want is a constant class (aka static) member that is the array. C++ does let you do this. Like so:
#include <iostream>
class A
{
public:
A();
static const int a[2];
};
const int A::a[2] = {0, 1};
A::A()
{
}
int main (int argc, char * const argv[])
{
std::cout << "A::a => " << A::a[0] << ", " << A::a[1] << "\n";
return 0;
}
The output being:
A::a => 0, 1
Now of course since this is a static class member it is the same for every instance of class A. If that is not what you want, ie you want each instance of A to have different element values in the array a then you're making the mistake of trying to make the array const to begin with. You should just be doing this:
#include <iostream>
class A
{
public:
A();
int a[2];
};
A::A()
{
a[0] = 9; // or some calculation
a[1] = 10; // or some calculation
}
int main (int argc, char * const argv[])
{
A v;
std::cout << "v.a => " << v.a[0] << ", " << v.a[1] << "\n";
return 0;
}
Where I've a constant array, it's always been done as static. If you can accept that, this code should compile and run.
#include <stdio.h>
#include <stdlib.h>
class a {
static const int b[2];
public:
a(void) {
for(int i = 0; i < 2; i++) {
printf("b[%d] = [%d]\n", i, b[i]);
}
}
};
const int a::b[2] = { 4, 2 };
int main(int argc, char **argv)
{
a foo;
return 0;
}
You can't do that from the initialization list,
Have a look at this:
http://www.cprogramming.com/tutorial/initialization-lists-c++.html
:)
A solution without using the heap with std::vector is to use boost::array, though you can't initialize array members directly in the constructor.
#include <boost/array.hpp>
const boost::array<int, 2> aa={ { 2, 3} };
class A {
const boost::array<int, 2> b;
A():b(aa){};
};
How about emulating a const array via an accessor function? It's non-static (as you requested), and it doesn't require stl or any other library:
class a {
int privateB[2];
public:
a(int b0,b1) { privateB[0]=b0; privateB[1]=b1; }
int b(const int idx) { return privateB[idx]; }
}
Because a::privateB is private, it is effectively constant outside a::, and you can access it similar to an array, e.g.
a aobj(2,3); // initialize "constant array" b[]
n = aobj.b(1); // read b[1] (write impossible from here)
If you are willing to use a pair of classes, you could additionally protect privateB from member functions. This could be done by inheriting a; but I think I prefer John Harrison's comp.lang.c++ post using a const class.
interestingly, in C# you have the keyword const that translates to C++'s static const, as opposed to readonly which can be only set at constructors and initializations, even by non-constants, ex:
readonly DateTime a = DateTime.Now;
I agree, if you have a const pre-defined array you might as well make it static.
At that point you can use this interesting syntax:
//in header file
class a{
static const int SIZE;
static const char array[][10];
};
//in cpp file:
const int a::SIZE = 5;
const char array[SIZE][10] = {"hello", "cruel","world","goodbye", "!"};
however, I did not find a way around the constant '10'. The reason is clear though, it needs it to know how to perform accessing to the array. A possible alternative is to use #define, but I dislike that method and I #undef at the end of the header, with a comment to edit there at CPP as well in case if a change.