string_view Vs const char* performance

November 30, 2021

Is a std::string_view parameter better than a const char* one in the code below?

void func( const std::string_view str )
{
    std::istringstream iss( str.data( ) ); // str is passed to the ctor of istringstream


    std::size_t pos { };
    int num { std::stoi( str.data( ), &pos, 10 ) }; // and here it's passed to std::stoi
}

int main()
{
    std::array<char, 20> buffer { };
    std::cin.getline( buffer.data( ), 20 );
    func( buffer.data( ) );
}

Both std::istringstream ctor and std::stoi require a const std::string& as their parameter. But I pass them a std::string_view object using its data() member function. Is this bad practice? Should I revert back to const char*?

>Solution :

But I pass them a std::string_view object using its data() member function. Is this bad practice

Yes, this is a bad practice. It’s bad primarily because a string view doesn’t necessarily point to a string that is null terminated. In case it doesn’t, passing data() into a function that requires null termination will result in undefined behaviour.

Secondarily, there are cases where knowing the length of the string beforehand is more efficient. The length is known since it’s stored in the string view. When you use data() only as an argument, you’re not providing the known size to the function.

Use this instead: std::istringstream iss(std::string{str});