c++ - Promotion of parameters during overloading -

i studying overloading , getting confused promotions. looked @ few articles in (implicit conversion sequence in function overloading) , sure more available, not find right article. referring http://www.dcs.bbk.ac.uk/~roger/cpp/week20.htm. looking @ c++ programming special edition stroustrup , came across following explanation.

finding right version call set of overloaded functions done looking best match between type of argument expression , parameters (formal arguments) of functions. approximate our notions of reasonable, series of criteria tried in order: 1 exact match [2] match using promotions; [3] match using standard conversions [4] match using user-defined conversions [5] match using ellipsis ......

void print(int); void print(double); void print(long); void print(char); void h(char c, int i, short s, float f) {     print(s); // integral promotion: invoke print(int)     print(f); // float double promotion: print(double) } 

i wrote below code. thinking if call function value of 1, func1(long) called because promotion takes place. error message "error: call of overloaded 'func1(int)' ambiguous". not calling function unsigned char type of variable.

also if pass call func1(3.4f), func1(double) called , promotion takes place per expectation. why 1 not promoted long int why float promoted double? integer promotions takeplace?

    void func1(unsigned char speed)     {         cout<<"func1 unsigned char: speed =" << speed <<" rpm\n";     }     void func1(long speed)     {         cout<<"func1 long int: speed =" << speed <<" rpm\n";     }      void func1(double speed)     {         cout<<"func1 double: speed =" << speed <<" rpm\n";     }      int main(void)     {         func1(1);         func1(3.4f);         return(0);     } 

the standard specifies:

[c++11: 4.13/1]: ("integer conversion rank")

every integer type has integer conversion rank defined follows:

• [..]
• the rank of long long int shall greater the rank of long int, shall greater rank of int, shall greater rank of short int, shall greater rank of signed char.
• the rank of unsigned integer type shall equal rank of corresponding signed integer type.
• [..]

which calls ambiguity in example.

as func1(3.4f);, it's promotion float double, , that's best match, since other 2 overloaded methods have long , unsigned char.

also check table:

where subclause specifies:

[conv.fpprom]: (7.7 floating-point promotion )

• a prvalue of type float can converted prvalue of type double. value unchanged.
• this conversion called floating-point promotion.