String representation of std::vector

Question

Is it possible to implement a C++ function which gives a string representation of every std::vector<T>, as long as the element of type T can be appended to an output stream like

T x;
...
std::cout << x << std::endl;

The string representation should look like

[x, y, z]

I've attempted the following, but what should ? be?

template <typename T> std::string vectorToString(std::vector<T>& vec) {
    std::string s;
    for (T element : vec) {
        ?
    }
    return s;
}

See http://stackoverflow.com/questions/4850473/pretty-print-c-stl-containers — Dan, Jan 11 '13 at 13:53
If you are interested in implementing this on your own, you probably want to learn about SFINAE, trailing decltype() and std::enable_if :) But it is doable, and it requires the tools above in order to detect at compile time if T is streamable. — bluescarni, Jan 11 '13 at 13:56
I thinkl it would be better to change signature to const ref: vectorToString( **const** std::vector< T > & vec ) — borisbn, Jan 11 '13 at 13:57
@bluescarni:I don't see why that would be necessary. If `T` isn't streamable the compiler would complain with an error somewhere in the function that it hasn't found any applicable `operator<<`. If you remove the function from the overload set for those cases it would complain that no applicable overload exists. So the only "advantage" of using `enable_if` would be a different, but probably worse, error-message. And why would you want to use trailing return? The test for streamability would best be written as a type trait anyways, so no need to use the parameters in the return type. — Grizzly, Jan 11 '13 at 14:27

Mike Seymour · Accepted Answer · 2013-01-11T14:27:55.553

4

You'll want a stringstream to do the formatting:

std::ostringstream ss;
ss << '['
bool first = true;
for (T const & element : vec) {
    if (!first) {
        ss << ", ";
    }
    ss << element;
    first = false;
}
ss << ']';
return ss.str();

edited Jan 11 '13 at 14:27

answered Jan 11 '13 at 13:56

Mike Seymour

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+1, but you probably want to take the element by reference to avoid unnecessary copying (since `operator<<` should never modify the original object): `for (T const & element : vec)`. – David Rodríguez - dribeas Jan 11 '13 at 14:27
@DavidRodríguez-dribeas: Indeed; I copied that line from the question without applying my brain. – Mike Seymour Jan 11 '13 at 14:31

score 1 · Answer 2 · answered Jan 11 '13 at 13:55

1

Use a std::ostringstream instance for ? and return std::ostringstream::str():

std::ostringstream s;
s << "[";

for (auto i(vec.begin()); i != vec.end(); i++)
{
    if (vec.begin() != i) s << ", ";
    s << *i;
}
s << "]";

return s.str();

answered Jan 11 '13 at 13:55

hmjd

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Andy Prowl · Answer 3 · 2013-01-11T14:24:44.190

If you are working on C++11, you can use this simple version:

#include <sstream>
#include <algorithm>

using namespace std;

template<typename T>
string format(vector<T> const& v)
{
    if (v.empty()) return "[]";
    ostringstream ss;
    ss << "[" << v[0];
    for_each(begin(v) + 1, end(v), [&ss] (T const& s) { ss << ", " << s; });
    ss << "]";
    return ss.str();
}

If you want to make it generic for other types of collections (not just vector) or even for sub-ranges of a collection, you can generalize it this way:

#include <sstream>
#include <algorithm>

using namespace std;

template<typename It>
string format(It b, It e)
{
    if (b == e) return "[]";
    ostringstream ss;
    ss << "[" << *b;
    for_each(++b, e, [&ss] (decltype(*b)& s) { ss << ", " << s; });
    ss << "]";
    return ss.str();
}

template<typename C>
string format(C const& c)
{
    return format(begin(c), end(c));
}

int main()
{
    vector<int> v = { 4, 5, 5, 8 };
    cout << format(v) << endl;
    return 0;
}

borisbn · Answer 4 · 2013-01-11T14:35:16.957

A little bit faster version

template <typename T> std::string vectorToString( const std::vector<T>& vec ) {
    if ( vec.empty() ) {
        return "[]";
    }
    std::ostringstream s;
    s << "[" << vec.front();

    for (auto i = vec.begin() + 1, e = vec.end(); i != e; i++)
    {
        s << ", " << *i;
    }
    s << "]";

    return s.str();
}

Another moment: maybe it would be right to specialize this function for strings and quote them, because if a string in vector begins or ends with , it would be hard to understand how many strings was printed.

template <>
std::string vectorToString< std::string >( const std::vector<std::string>& vec ) {
    if ( vec.empty() ) {
        return "[]";
    }
    std::ostringstream s;
    s << "[" << vec.front();

    for (auto i = vec.begin() + 1, e = vec.end(); i != e; i++)
    {
        s << ", \"" << *i << "\"";
    }
    s << "]";

    return s.str();
}

But what if the element strings contain quotes? Generation of CSV is a bit more complicated and one should use an existing library. — Ben Voigt, Jan 11 '13 at 14:48

score 0 · Answer 5 · answered Jan 11 '13 at 14:33

Using algorithms, which usually simplify code (not sure if in this case, but added for the sake of completion):

template <typename T>
std::string toString( std::vector<T> const & v ) {
   if (v.empty()) 
      return "[]";
   typename std::vector<T>::const_iterator last = std::prev(v.end());
   std::ostringstream st;
   st << "[ ";
   std::copy( v.begin(), last, std::ostream_iterator<T>(st,", ") );
   st << *last << " ]";
   return st.str();
}

String representation of std::vector

5 Answers5