14 Templates [temp]

14.3 Template arguments [temp.arg]

14.3.2 Template non-type arguments [temp.arg.nontype]

A template-argument for a non-type, non-template template-parameter shall be one of:

  • for a non-type template-parameter of integral or enumeration type, a converted constant expression ([expr.const]) of the type of the template-parameter; or

  • the name of a non-type template-parameter; or

  • a constant expression ([expr.const]) that designates the address of a complete object with static storage duration and external or internal linkage or a function with external or internal linkage, including function templates and function template-ids but excluding non-static class members, expressed (ignoring parentheses) as & id-expression, where the id-expression is the name of an object or function, except that the & may be omitted if the name refers to a function or array and shall be omitted if the corresponding template-parameter is a reference; or

  • a constant expression that evaluates to a null pointer value ([conv.ptr]); or

  • a constant expression that evaluates to a null member pointer value ([conv.mem]); or

  • a pointer to member expressed as described in [expr.unary.op]; or

  • a constant expression of type std::nullptr_t.

Note: A string literal ([lex.string]) does not satisfy the requirements of any of these categories and thus is not an acceptable template-argument. [ Example:

template<class T, const char* p> class X {
  /* ... */

X<int, "Studebaker"> x1;        // error: string literal as template-argument

const char p[] = "Vivisectionist";
X<int,p> x2;                    // OK

 — end example ]  — end note ]

Note: Addresses of array elements and names or addresses of non-static class members are not acceptable template-arguments. [ Example:

template<int* p> class X { };

int a[10];
struct S { int m; static int s; } s;

X<&a[2]> x3;                    // error: address of array element
X<&s.m> x4;                     // error: address of non-static member
X<&s.s> x5;                     // error: &S::s must be used
X<&S::s> x6;                    // OK: address of static member

 — end example ]  — end note ]

Note: Temporaries, unnamed lvalues, and named lvalues with no linkage are not acceptable template-arguments when the corresponding template-parameter has reference type. [ Example:

template<const int& CRI> struct B { /* ... */ };

B<1> b2;                        // error: temporary would be required for template argument

int c = 1;
B<c> b1;                        // OK

 — end example ]  — end note ]

The following conversions are performed on each expression used as a non-type template-argument. If a non-type template-argument cannot be converted to the type of the corresponding template-parameter then the program is ill-formed.


template<const int* pci> struct X { /* ... */ };
int ai[10];
X<ai> xi;                       // array to pointer and qualification conversions

struct Y { /* ... */ };
template<const Y& b> struct Z { /* ... */ };
Y y;
Z<y> z;                         // no conversion, but note extra cv-qualification

template<int (&pa)[5]> struct W { /* ... */ };
int b[5];
W<b> w;                         // no conversion

void f(char);
void f(int);

template<void (*pf)(int)> struct A { /* ... */ };

A<&f> a;                        // selects f(int)

 — end example ]