Consider this pseudo code for a type deduction case:
template<typename T> void f(ParamType param);
Call to function will be:f(expr);
According to type deduction case where ParamType is not a reference, pointer, nor a universal reference (see S. Meyers "Effective Modern C++", p.14), but passed by value, to determine type T, one needs firstly to ignore the reference and const part of 'expr' and then pattern-match exprs type to determine T.
The driver will be:
void PerformTest() {
int i = 42;
int* pI = &i;
f_const_left(pI);
f_non_template_left(pI);
f_const_right(pI);
f_non_template_right(pI);
}
Now consider these functions, which, using this deduction rule, are showing some counter-intuitive results while being called with pointer as an argument:
template<typename T> void f_const_left(const T t) {
// If 'expr' is 'int *' then, according to deduction rule for value parameter (Meyers p. 14),
// we need to get rid of '&' and 'const' in exp (if they exist) to determine T, thus T will be 'int *'.
// Hence, ParamType will be 'const int *'.
// From this it follows that:
// 1. This function is equivalent to function 'func(const int * t){}'
// 2. If ParamType is 'const int *' then we have non-const pointer to a const object,
// which means that we can change what pointer points to but cant change the value
// of pointer address using operator '*'
*t = 123;// compiler shows no error which is contradiction to ParamType being 'const int *'
t = nullptr; // compiler shows error that we cant assign to a variable that is const
// As we see, consequence 2. is not satisfied:
// T is straight opposite: instead of being 'const int *'
// T is 'int const *'.
// So, the question is:
// Why T is not 'const int*' if template function is f(const T t) for expr 'int *' ?
}
Consider consequence 1.:
Lets create an equivalent non-template function:
void f_non_template_left(const int* t) {
// 1. Can we change the value through pointer?
*t = 123; // ERROR: expression must be a modifiable lvalue
// 2. Can we change what pointers points to?
t = nullptr; // NO ERROR
// As we can see, with non-template function situation is quite opposite.
}
For for completeness of the experiment, lets also consider another pair of functions but with 'const' being placed from the right side of a T: one template function and its non-template equivalent:
template<typename T> void f_const_right(T const t) {
// For expr being 'int *' T will be 'int *' and ParamType will be 'int * const',
// which is definition of a constant pointer, which cant point to another address,
// but can be used to change value through '*' operator.
// Lets check it:
// Cant point to another address:
t = nullptr; // compiler shows error that we cant assign to a variable that is const
// Can be used to change its value:
*t = 123;
// So, as we see, in case of 'T const t' we get 'int * const' which is constant pointer, which
// is intuitive.
}
Finally, the non-template function with 'const' from the right side of type:
void f_non_template_right(int* const t) {
// 1. Can we change the value through pointer?
*t = 123; // No errors
// 2. Can we change what pointers points to?
t = nullptr; // ERROR: you cant assign to a variable that is const
// As we can see, this non-template function is equivalent to its template prototype
}
Can someone explain why there is such insonsistency between template and non-template functions ? And why template function with 'const' on the left is behaving not according to the rule of deduction?
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