All the vector code for this article is in the main() function, unless otherwise indicated. Vector of pointers to different types, is however, addressed at the end of the article. In order to appreciate vector-of-pointers, it is good to recall the knowledge for vector of objects.
Article Content
- Recall for Vector-of-objects
- Vector of Pointers of Same Type
- Vector of Pointers to Different Types
- Conclusion
Recall for Vector of Objects
Vector of Character Example
The following program, shows an example of a vector of chars:
#include <vector>
using namespace std;
int main()
{
vector vtr = {'U', 'V', 'W', 'X', 'Y', 'Z'};
for (int i=0; i<vtr.size(); i++)
cout << vtr[i] << ' ';
cout << endl;
for (vector::iterator it = vtr.begin(); it != vtr.end(); it++)
cout << *it << ' ';
cout << endl;
return 0;
}
The output is:
U V W X Y Z
The same list has been displayed twice. The first statement in the main() function, creates a vector of chars. The next two code segments, print out the same vector list at the terminal. The first of these code segments, uses indexes. The second uses iterators.
Vector of Integer Example
The following program, shows an example of a vector of ints:
#include <vector>
using namespace std;
int main()
{
vector vtr = {1, 2, 3, 4, 5, 6};
for (int i=0; i<vtr.size(); i++)
cout << vtr[i] << ' ';
cout << endl;
for (vector::iterator it = vtr.begin(); it != vtr.end(); it++)
cout << *it << ' ';
cout << endl;
return 0;
}
The output is:
1 2 3 4 5 6
The same list has been displayed twice, in the same way, as in the previous code.
Vector of String Object Example
A const pointer to chars, points to a string literal. A string object is instantiated from the string class. To have a string object, the string library has to be included with a directive, at the top of the program. It is possible and easy to have a vector of string objects, as the following program shows:
#include <vector>
#include <string>
using namespace std;
int main()
{
vector vtr = {"one", "two", "three", "four", "five"};
for (int i=0; i<vtr.size(); i++)
cout << vtr[i] << ' ';
cout << endl;
for (vector::iterator it = vtr.begin(); it != vtr.end(); it++)
cout << *it << ' ';
cout << endl;
return 0;
}
The output is:
one two three four five
The same list has been displayed twice, in the same way, as in the previous code.
Vector of Instantiated Object Example
The programmer can create his own class, instantiate objects from the class, and then put the objects in a vector. The following program illustrates this:
#include <vector>
using namespace std;
class TheCla {
public:
const char* str;
TheCla (char chs[]) {
str = chs;
}
};
int main()
{
char ch1[] = "text1", ch2[] = "text2", ch3[] = "text3", ch4[] = "text4", ch5[] = "text5";
TheCla obj1(ch1), obj2(ch2), obj3(ch3), obj4(ch4), obj5(ch5);
vector vtr = {obj1, obj2, obj3, obj4, obj5};
for (int i=0; i<vtr.size(); i++)
cout << vtr[i].str << ' ';
cout << endl;
for (vector::iterator it = vtr.begin(); it != vtr.end(); it++)
cout <str << ' ';
cout << endl;
return 0;
}
The output is:
text1 text2 text3 text4 text5
The same list has been displayed twice, in the same way, as in the previous code.
The class has a constructor and one public data member. The constructor assigns its argument to this data member.
The values of the vector, this time, are string literals, and not string objects. So, note the way the literals and vector elements have been coded.
If an object instantiated from the class is obj, then the value of the member, str would be accessed through the object, as:
obj in this case is a reference. Note the use of the dot operator. This is why, in the last-but-one code segment, in the main() function, each vector value has been accessed as:
where [i] is the index.
If a pointer to obj is “it”, then the value of the member, str would be accessed through the object, as:
Note the use of the arrow operator here. The iterator is like a pointer. This is why, in the last code segment, each vector value has been accessed as:
where “it” is the iterator.
Vector of Pointers of Same Type
Example for Vector of Pointers to Characters
The following program, shows an example of a vector of pointers to chars:
#include <vector>
using namespace std;
int main()
{
char ch1 = 'U', ch2 = 'V', ch3 = 'W', ch4 = 'X', ch5 = 'Y', ch6 = 'Z';
vector vtr = {&ch1, &ch2, &ch3, &ch4, &ch5, &ch6};
for (int i=0; i<vtr.size(); i++)
cout << *vtr[i] << ' ';
cout << endl;
for (vector::iterator it = vtr.begin(); it != vtr.end(); it++)
cout << **it << ' ';
cout << endl;
return 0;
}
The output is:
U V W X Y Z
The same list has been displayed twice. The first statement in the main() function creates 6 characters with their identifiers. The second statement represents these characters, with their addresses in memory; and that results in a vector of pointers to chars. Note the template argument of this vector. The next two code segments, print out the same vector list at the terminal. The first of these code segments uses indexes. The second uses iterators.
For the first of these code segments, since each element in the vector is a pointer, the index reference has to be dereferenced by the indirection operator, *.
The iterator is like a pointer. For the second of these code segments, since each element in the vector is a pointer, and the iterator is like a pointer, each element is seen as a pointer to pointer. And so each element has to be dereferenced twice, with **.
Example of Vector of Pointers to Integers
The following program, which is similar to the above, shows an example of a vector of pointers to ints:
#include <vector>
using namespace std;
int main()
{
int int1 = 1000, int2 = 2000, int3 = 3000, int4 = 4000, int5 = 5000, int6 = 6000;
vector vtr = {&int1, &int2, &int3, &int4, &int5, &int6};
for (int i=0; i<vtr.size(); i++)
cout << *vtr[i] << ' ';
cout << endl;
for (vector::iterator it = vtr.begin(); it != vtr.end(); it++)
cout << **it << ' ';
cout << endl;
return 0;
}
The output is:
1000 2000 3000 4000 5000 6000
The same list has been displayed twice, in the same way, as of the previous code.
Example of Vector of Pointers to String Objects
A const pointer to chars, points to a string literal. A string object is instantiated from the string class. To have a string object, the string library has to be included with a directive, at the top of the program. It is possible and easy to have a vector of pointers to string objects, as the following program shows:
#include <vector>
#include <string>
using namespace std;
int main()
{
string str1 = "aaa", str2 = "bbb", str3 = "ccc", str4 = "ddd", str5 = "eee", str6 = "fff";
vector vtr = {&str1, &str2, &str3, &str4, &str5, &str6};
for (int i=0; i<vtr.size(); i++)
cout << *vtr[i] << ' ';
cout << endl;
for (vector::iterator it = vtr.begin(); it != vtr.end(); it++)
cout << **it << ' ';
cout << endl;
return 0;
}
The output is:
aaa bbb ccc ddd eee fff
The same list has been displayed twice, in the same way, as of the previous code.
Vector of Pointers to User Defined Class Objects
The programmer can create his own class, instantiate objects from the class, and then put pointers to the objects, in a vector. The following program illustrates this:
#include <vector>
using namespace std;
class TheCla {
public:
const char* str;
TheCla (char chs[]) {
str = chs;
}
};
int main()
{
char ch1[] = "text1", ch2[] = "text2", ch3[] = "text3", ch4[] = "text4", ch5[] = "text5";
TheCla obj1(ch1), obj2(ch2), obj3(ch3), obj4(ch4), obj5(ch5);
vector vtr = {&obj1, &obj2, &obj3, &obj4, &obj5};
for (int i=0; i<vtr.size(); i++)
cout <str << ' ';
cout << endl;
for (vector::iterator it = vtr.begin(); it != vtr.end(); it++)
cout <str << ' ';
cout << endl;
return 0;
}
The output is:
text1 text2 text3 text4 text5
The same list has been displayed twice, in the same way, as in the previous code.
The class has a constructor and one public data member. The constructor assigns its argument to this data member.
The values of the vector, this time, are string literals, and not string objects. So, note the way the literals and vector elements, have been coded.
If an object instantiated from the class is obj, then the value of the member, str would be accessed through the object, as:
obj in this case is a reference. If a pointer to obj is ptr, then the value of the member, str would be accessed through the pointer, as:
This is why in the last-but-one code segment, each vector value has been accessed as:
If ptrptr is a pointer to ptr (pointer to pointer), the value of the member, str would be accessed through the pointer, as:
The parentheses ensure that (*ptrptr) is evaluated first, instead of the possible first evaluation of (ptrptr->str).
The iterator is like a pointer. This is why in the last code segment, each vector value has been accessed as:
where “it” is the iterator.
Vector of Pointers to Different Types
To have a vector of pointers to different types, use the following procedure:
- Let the template of the vector, be pointer to void.
- Let the values of the vectors, be addresses of the different objects of different types.
- When reading out the values, cast the void pointers to their appropriate types.
The following program illustrates these, with a char, int and string objects:
#include <vector>
#include <string>
using namespace std;
int main()
{
char ch = 'U';
int inte = 1000;
string str = "I love you.";
vector vtr = {&ch, &inte, &str};
cout << *((char*)vtr[0]) << endl;
cout << *((int*)vtr[1]) << endl;
cout << *((string*)vtr[2]) << endl;
return 0;
}
The output is:
1000
I love you.
Conclusion
A vector of pointers is similar to a vector of objects. The main differences are as follows: The values of the vector of pointers, have to be addresses of objects declared from or instantiated from the class. Assume that the class name is TheCla, then the template argument of the vector has to be “TheCla*”. The address of an object is obtained by preceding the identifier of the object, with & .
