Returning vector from template function - c++

Question

Hoping for some clarification here. The code below executes fine, but when I uncomment that else statement a compilation error occurs. It's because in main I'm specifying a type int event though there's the possibility of type string. I've simplified my actual code to what's below to narrow down on the problem, what can I do to make it so that vector data in main can be of whatever type getNextLineOfData returns?
#include <vector>
using namespace std;
template< typename T>
std::vector<T> getNextLineOfData(bool someBoolean)
{
std::vector<T> data;
if (someBoolean)
{
data.push_back(1);
data.push_back(2);
data.push_back(3);
}
/*
else
{
data.push_back("1");
data.push_back("2");
data.push_back("3");
}
*/
return data;
};
int main()
{
vector<int> data = getNextLineOfData<int>(true);
return 0;
}

Answer 1

You are confusing compile time operations with runtime operations in your code snippet. When you template the function getNextLineOfData and instantiate it with getNextLineOfData<int>, the compiler goes ahead and generates a function that returns a vector for you. The if statement however is only evaluated at run time. So when the compiler tries to build your code it sees that you are adding both 1 and "1" to your vector<int> container based on the conditional. This is not allowed.
You could solve your problem with template specialization.
#include <vector>
using namespace std;
template<typename T>
std::vector<T> getNextLineOfData() {
// default
}
template<>
std::vector<int> getNextLineOfData()
{
std::vector<int> data;
data.push_back(1);
data.push_back(2);
data.push_back(3);
return data;
};
template<>
std::vector<std::string> getNextLineOfData()
{
std::vector<std::string> data;
data.push_back("1");
data.push_back("2");
data.push_back("3");
return data;
};
int main()
{
vector<int> data = getNextLineOfData<int>();
return 0;
}
EDIT: As #BobTFish points out, it might be better to overload the function rather than template specialize it. The solution above solves the problem the way you had it initially set up.

Answer 2

Reading from extra information in comments, I would suggest something like:
void getNextLine(std::vector<std::string>& output)
{
output.push_back("string data as you please");
}
void getNextLine(std::vector<int>& output)
{
output.push_back(1);
}
bool nextLineIsIntData()
{
// somehow determine if the coming data is strings or ints
return false;
}
int main()
{
std::vector<std::string> stringData;
std::vector<int> intData;
if (nextLineIsIntData())
getNextLine(intData);
else
getNextLine(stringData);
// do whatever you want
}

Answer 3

Well what you are doing is simply illegal. When you look at the if-else statement you say, well if some condition is true than this will execute but this won't, so it stands too reason that the compiler will ignore the part that is not executed. This is flat out wrong. What you need to do, which is layed out in previous answers is too overload or specialize the function for the different data types.
I should also mention that what you are trying to do is bad style. You are essentially relying on the user too pass the correct bool value, which influences the types you push_back() into the vector. Why do this when you have the power of template pattern matching at your disposal which completely removes the need to rely on correct user input.
In this case and any similar ones you come across it's much better to let the compiler decide