C++ template argument type deduction

Question

Given a templated function declared like this:

template
int Function(T object);

A user can invoke this function by specifying th

Answer 1

Note: The answer here has been borrowed from effective modern C++ with a (very) few additions of my own

This is one of those questions that are easy to pose but difficult to answer! I remember reading an entire ch. apter on template type deduction, and to a rookie reader, the answer is not clear in one read either. Nevertheless, I will try to clarify it here.

One should note that there is something called Universal References (which are not the same as references or r-value references) that influences template type deduction, and I assume readers know about l-value and r-value references.

Any ubiquitous function template definition looks like the following:

template 
returnType function(paramType param);

A call to function would look somehow look like this :

function(expression);

The compiler uses expression to determine the type of T and the type of paramType. This is so because more often paramType contains decorations like const, const&, const&&, etc. Beginners would be tempted to believe that the type T deduced by the compiler will be the same as the type of expression, i.e., the argument passed to the function, but it is not always the case. Deduction of type T depends both on expression and paramType . Depending on what the function parameter paramType is there are three cases to be considered for template type deduction:

1. paramType is pointer or reference but not a universal reference.
2. paramType is a universal reference.
3. paramType is neither a pointer nor a reference.

Let's take a look at each case one by one

Case 1: paramType is a pointer or a reference but not a universal reference

Call me crazy, but this is the simplest case that can be encountered. In this case, type deduction works like this: (i) If expression is a reference, then ignore the reference part (ii) then match expression's pattern against paramType to determine T

Lets take a look at an example :

template 
returnType function(T ¶m);

We have the following variable declarations:

int x = 23;               // x is int
const int const_x = x;    // const_x is const int
const int& ref_x = x;     // ref_x is a reference to x as const int

The deduced call for T and param in various calls are as follows :

f(x);                    //T is int, param's type is int&
f(const_x);              //T is const int, param's type is const int&
f(ref_x);                //T is const int, param's type is const int&

There are two points to be noted here:

(i) the compiler ignores the reference-ness for type deduction here

(ii) the const-ness becomes a part of type T when passing a const object or reference to a const object, and hence passing const objects or references to const object to functions taking parameter T& is safe.

If we change the function parameter from T& to const T&, because in this case we are assuming param to be reference to const, the const-ness need not be deduced as a part of T . Below is an example:

template 
returnType function(const T& param);  // param is now a ref-to-const

int x = 23;                    // same as previous
const int const_x = x;         // same as previous
const int& ref_x = x;          // same as previous

f(x);                         // T is int, paramType is const int&
f(const_x);                   // T is int, paramType is const int&
f(ref_x);                     // T is int, paramType is const int&

Note: variable 'x' is not a const argument to 'f()' but it is till deduced as a const param

If paramType is a pointer, things will work fundamentally the same way as with references. There will be pointers instead of references. E.g., below for the sake of completeness is provided:

template 
returnType function( T* paramType);  // paramType is now a pointer

int x = 23;                      // same as before
const int *pointer_x = &x;       // pointer_x is pointer to x as const int

f(&x);                          // T is int, paramType is int*
f(pointer_x);                   // T is const int, paramType is const int*

For the sake of completeness I may as well post the case if paramType were a pointer to a constant object like the following:

template 
returnType function(const T* paramType);

int x = 23;                      // same as before
const int *pointer_x = &x;       // pointer_x is pointer to x as const int

f(&x);                          // T is int, paramType is const int*
f(pointer_x);                  // T is int, paramType is const int*

i.e., again the const-ness is not anymore deduced as a part of T

In case of r-value references, type T and paramType deduction follow essentially the same rules as they do in case of l-value references.

This covers most of it for the first case. Let's look at our case 2.

Case 2: paramType is a universal reference

Universal references are declared like r-value references but take l-value, but what makes their behavior different is that the function arguments receive l-value references. Here's how the type deduction works for this case:

(i) If expression is an l-value, both T and paramType are deduced to be l-value. (This seems strange in the face of how the code looks like because although paramType is declared using the syntax of r-value reference, its deduced type is of l-value reference.) It should be noted that this is the only case where T is deduced to be a reference.

The example below clarifies my explanation:

template 
returnType function(T&& paramType);  // param becomes universal reference if
                                     // argument to function call is an l-value

int x = 23                     // same as previous
const int const_x = x;         // same as previous
const int& ref_x = x;          // same as previous

f(x);             // x is l-value therefore T is int&
                  // paramType is int&

f(const_x);       // const_x is l-value therefore T is const int&
                  //paramType is also const int&

f(ref_x);        // ref_x is l-value therefore T is const int&
                 // paramType is also const int&

f(23);          // 23 is r-value so T is int
                // paramType is now int&&

I want to be honest here and say that this doesn't explain why universal references work the way they do, but I think this post will become too lengthy if I go on to justify it here.

Case 3: paramType is neither a pointer nor a reference

This is where pass-by-value in template occurs, which implies that param will be a copy of whatever is passed to the argument of the calling function, i.e., a completely new object, and this motivates the rules that govern type deduction of T from expression . Two points to be noted here are:

(i) ignore the refrence-ness in expression , if there happens to be one.

(ii) after ignoring the ref-ness, ignore const-ness or volatile-ness too, i.e if present

template 
returnType function(T paramType);

int x = 23;
const int const_x = x;
const int& ref_x = x;

f(x);             // T and paramType are both int
f(const_x);       // T and paramType are both int here too
f(ref_x);         // T and paramType are both int again

Note even though const_x and ref_x are const objects which cannot be modified, it doesn't mean that their copies cannot be modified. This looks straightforward, but it gets tricker when we pass a constant pointer to a constant object. Let's take a look at another example:

template 
returnType function(T param);

const double *const dPtr = 23;  // dPtr is const pointer to const double

function(dPtr);             // passing argument of type const double *const

When const pointer is passed by value, the const-ness is lost, and the pointer is copied by value, which is in sync with the type deduction rules for pass by value, but the const-ness of what pointer points to is preserved, and hence the paramType will be const *double.

This might get your head spinning as it did to me when I started to learn about it. The best way would be to re-read it and try to code it.