工作函数指针模板给Netbeans错误

Question

I have a set of functions that take no parameters and return an integer, and I want to cache the result of them in C++03.

After refreshing my function pointer syntax, I write:

#include <iostream>

int return_one() {
    std::cout << "Calling one\n";
    return 1;
}
int return_two() {
    std::cout << "Calling two\n";
    return 2;
}
template <int(*f)()>       // line 11
int cached() {
    static int x = f();    // line 13
    return x;
}

int main() {
    std::cout << cached<return_one>() << '\n';
    std::cout << cached<return_two>() << '\n';
    std::cout << cached<return_one>() << '\n';
    std::cout << cached<return_two>() << '\n';
    std::cout << cached<return_one>() << '\n';
    std::cout << cached<return_two>() << '\n';
}

Which prints:

Calling 1
1
Calling 2
2
1
2
1
2

Everything looks good to me.

However, Netbeans underlines f in lines 11 and 13, complaining "Unable to resolve identifier f" as an error. I can click the "Run Project (F6)" button and the program happily compiles and runs.

Why does Netbeans complain about my template parameter?

---

For bonus points, the set of functions I have take no parameters, but some return int , and some return bool . I was just going to copy-paste my template, one for each function return type. Is there a single template I can use to handle both return types?

Answer 1

You can insert a typedef to get rid of the red line:

typedef int(*f);

template <int(*f)()>       // line 11
int cached() {
    static int x = f();    // line 13
    return x;
}

---

To get it working with other types:

typedef int(*f); // Keeps warning away - again

template <class T, T(*f)()>       // line 11
T cached() {
    static T x = f();    // line 13
    return x;
}

You have to add another template-argument though.

Examples:

bool return_bool()
{
    std::cout << "Calling bool\n";
    return true;
}

float return_float()
{
    std::cout << "Calling float\n";
    return 0.31f;
}

std::string return_string()
{
    std::cout << "Calling std::string\n";
    return "abc";
}

// ...

std::cout << cached<int, return_one>() << '\n';
std::cout << cached<int, return_two>() << '\n';
std::cout << cached<int, return_one>() << '\n';
std::cout << cached<int, return_two>() << '\n';
std::cout << cached<int, return_one>() << '\n';
std::cout << cached<int, return_two>() << '\n';

std::cout << cached<bool, return_bool>() << '\n';
std::cout << cached<bool, return_bool>() << '\n';
std::cout << cached<float, return_float>() << '\n';
std::cout << cached<float, return_float>() << '\n';
std::cout << cached<std::string, return_string>() << '\n';
std::cout << cached<std::string, return_string>() << '\n';

Output:

Calling one
1
Calling two
2
1
2
1
2
Calling bool
1
1
Calling float
0.31
0.31
Calling std::string
abc
abc