Template integer parameter constructor

Question

I don't understand the following constructor (part of Qt library in src\corelib\tools\qstringbuilder.h), what does it mean and how does it work?

class QLatin1Literal
{
public:
    int size() const { return m_size; }
    const char *data() const { return m_data; }

    template <int N>
    QLatin1Literal(const char (&str)[N])
        : m_size(N - 1), m_data(str) {}

private:
    const int m_size;
    const char * const m_data;
};

Answer 1

The constructor is taking string literal as an argument. What you see is just a syntax to declare a template for this.

With such constructor m_size can be found in O(1) opposed to O(strlen(str)) required otherwise with a non-template constructor taking char const* as an argument.

A thing to remember of is for each string length there will be an instance of template generated by compiler so you may end up having quite a few instantiations of this template in your library/binary/object files.

Answer 2

The constructor argument is a reference to an array of N characters. It initialises m_data to point to the first character, and m_size to one less than the size of the array.

A string literal, like "hello", is an array of characters containing those in the string, followed by a zero-valued terminator. So, if the constructor is called with one of those:

 QLatin1Literal lit("hello");
 assert(lit.size() == strlen("hello"));  // SUCCESS: m_size is inferred as 5

it will infer a value of 6 for N (since the array contains the five characters of "hello", plus the terminator), and initialise m_size to 5 (the actual string length).

Beware that this could go wrong if the array isn't actually a string literal; for example:

char buffer[1000] = "hello";  // array size is larger than string+terminator
QLatin1Literal lit(buffer);
assert(lit.size() == strlen("hello"));  // FAIL: m_size is inferred as 999