Typeid of vectors

This is a discussion on Typeid of vectors within the C++ Programming forums, part of the General Programming Boards category; Here is a code I wrote to check the type of a variable in run time. In this case I ...

  1. #1
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    Typeid of vectors

    Here is a code I wrote to check the type of a variable in run time. In this case I am trying to get the type info of a vector.

    But when I run this program I get a cryptic string from which it cannt be derived as which type of vector it is.

    Code:
    #include <typeinfo>
    
    using namespace std;
    
    //register uint32_t AVP_LENGTH_VALUE = htonl(0x00FFFFFF);
    
    int main()
    {
            vector<int> list;
    
            cout << typeid(list).name() << endl;
    }
    The output is
    Code:
    St6vectorIiSaIiEE
    Please suggest a way to decode this.

  2. #2
    C++ Witch laserlight's Avatar
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    Why do you want to do this?
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  3. #3
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    Quote Originally Posted by laserlight View Post
    Why do you want to do this?
    Seconded. But in case the reason is valid why not do:

    Code:
        std::vector<int> abc;    
        if(typeid(std::vector<int>) == typeid(abc))
        {
            std::cout<<"Matching Type";
        }//if
    Woop?

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    Code:
    #include <vector>
    A good idea when using vectors.

  5. #5
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    Quote Originally Posted by laserlight View Post
    Why do you want to do this?
    I would like to perform some operation on the variable depending on the type of variable.

    For example:
    writing a toString() implementation, that will convert the object into string representation.

    if(type == "vector<int>")
    {

    }

    if(type == "vector<SomeClass>")
    {

    }

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by sanjivko
    I would like to perform some operation on the variable depending on the type of variable.
    Ah, but then manually checking the variable's type should be a last resort.

    Quote Originally Posted by sanjivko
    For example:
    writing a toString() implementation, that will convert the object into string representation.
    Write a function template instead.
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  7. #7
    The larch
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    These kinds of things are handled by templates. Runtime type information will be completely useless for this purpose.

    For example, a function to turn a vector into a string representation might look like this:

    Code:
    #include <iostream>
    #include <sstream>
    #include <vector>
    
    template <class T>
    std::string to_string(const std::vector<T>& vec)
    {
        std::stringstream result;
        result << "(";
        if (!vec.empty()) {
            result << vec[0];
            for (std::size_t i = 1; i != vec.size(); ++i) {
                result << ", " << vec[i];
            }
        }
        result << ")";
        return result.str();
    }
    
    int main()
    {
        std::vector<int> vec;
        std::cout << to_string(vec) << '\n';           // ()
        vec.push_back(1);
        std::cout << to_string(vec) << '\n';           // (1)
        std::vector<std::string> string_vec;
        string_vec.push_back("Hello");
        string_vec.push_back("world");
        std::cout << to_string(string_vec) << '\n';    //(Hello, world)
    }
    This relies on ostream operator<< being overloaded for the type T (the standard library way to convert something to its string representation).

    If you need to use a variable of type T, then you know what it is: T.

    Code:
    template <class T>
    void foo(const std::vector<T>& vec)
    {
         T first = vec.front();
         ...
    }
    Last edited by anon; 10-21-2010 at 02:48 AM.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  8. #8
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by anon
    For example, a function to turn a vector into a string representation might look like this:
    I think phantomap once mentioned to me that an implementation is permitted additional template parameters for the standard containers, so even if we account for the allocator, it is still a little iffy as to how portable this might be. However, we could write:
    Code:
    template <class T>
    std::string to_string(const T& container)
    {
        std::stringstream result;
        result << "(";
        if (!container.empty()) {
            typedef typename T::const_iterator iterator_type;
            iterator_type i = container.begin();
            result << *i;
            ++i;
            for (const iterator_type end = container.end(); i != end; ++i) {
                result << ", " << *i;
            }
        }
        result << ")";
        return result.str();
    }
    EDIT:
    As a bonus, I expect that this will not only work for std::vector, but also std::deque, std::list, and even std::set and std::multiset.
    Last edited by laserlight; 10-21-2010 at 03:07 AM.
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  9. #9
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    Smile

    Quote Originally Posted by anon View Post
    These kinds of things are handled by templates. Runtime type information will be completely useless for this purpose.

    For example, a function to turn a vector into a string representation might look like this:

    Code:
    #include <iostream>
    #include <sstream>
    #include <vector>
    
    template <class T>
    std::string to_string(const std::vector<T>& vec)
    {
        std::stringstream result;
        result << "(";
        if (!vec.empty()) {
            result << vec[0];
            for (std::size_t i = 1; i != vec.size(); ++i) {
                result << ", " << vec[i];
            }
        }
        result << ")";
        return result.str();
    }
    
    int main()
    {
        std::vector<int> vec;
        std::cout << to_string(vec) << '\n';           // ()
        vec.push_back(1);
        std::cout << to_string(vec) << '\n';           // (1)
        std::vector<std::string> string_vec;
        string_vec.push_back("Hello");
        string_vec.push_back("world");
        std::cout << to_string(string_vec) << '\n';    //(Hello, world)
    }
    This relies on ostream operator<< being overloaded for the type T (the standard library way to convert something to its string representation).

    If you need to use a variable of type T, then you know what it is: T.

    Code:
    template <class T>
    void foo(const std::vector<T>& vec)
    {
         T first = vec.front();
         ...
    }

    Anon, thanks for the reply this is what I exactly needed.

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