Be cautious when creating c++ template functions

included in C++

2023-01-16 One minute

When creating C++ template functions, the type-specified version must always be declared before the template version. Which means,

#include <vector>
#include <iostream>
using namespace std;
template<typename T>
void foo(const std::vector<T> & data)
{
    cout<<"foo template"<<endl;
    std::vector<int> data2(data.begin(),data.end());
    foo(data2);
}
void foo(const std::vector<int> & data)
{
    cout<<"foo int"<<endl;
}
int main()
{
    std::vector dvec{0.0,1.0};
    foo(dvec);
    return 0;
}

Declaring and using it this way (calling foo results in the same outcome) produces the following result:

...
foo template
foo template
foo template
foo template
foo template
foo template
Segmentation fault (core dumped)

This occurs because there is no function declared before the template for the compiler to find the type-specified version. Since templates inherently contain their own implementation, declarations of other functions must come first. By simply changing the order, the following code will execute correctly.

#include <vector>
#include <iostream>
using namespace std;
void foo(const std::vector<int> & data)
{
    cout<<"foo int"<<endl;
}
template<typename T>
void foo(const std::vector<T> & data)
{
    cout<<"foo template"<<endl;
    std::vector<int> data2(data.begin(),data.end());
    foo(data2);
}

int main()
{
    std::vector dvec{0.0,1.0};
    foo(dvec);
    return 0;
}

Result

foo template
foo int