Define instantiation.
Class or function generated by the compiler from a template.
Write and test your own versions of the
comparefunctions.
Call your
comparefunction on twoSales_dataobjects to see how your compiler handles errors during instantiation.
error C2678: binary '<': no operator found which takes a left-hand operand of type 'const Sales_data' (or there is no acceptable conversion)Write a template that acts like the library
findalgorithm. The function will need two template type parameters, one to represent the function’s iterator parameters and the other for the type of the value. Use your function to find a given value in avector<int>and in alist<string>.
Write a template version of the
How do you think the library
beginandendfunctions that take an array argument work? Define your own versions of these functions.
Write a
constexprtemplate that returns the size of a given array.
In the “Key Concept” box on page 108, we noted that as a matter of habit C++ programmers prefer using != to using <. Explain the rationale for this habit.
As we’ve seen, only a few library types, vector and string being among them, have the subscript operator. Similarly, all of the library containers have iterators that define the == and != operators. Most of those iterators do not have the < operator. By routinely using iterators and !=, we don’t have to worry about the precise type of container we’re processing.
What is a function template? What is a class template?
- function template: Definition from which specific functions can be instantiated.
- class template: Definition from which specific classes can be instantiated.
- differ: the compiler cannot deduce the template parameter type(s) like function templates for a class template. we should supply the list of template arguments to use in place of the template parameters inside angle brackets following the template's name.
What happens when a class template is instantiated?
the compiler rewrites the class template, replacing each instance of the template parameter T by the given template argument.
The following definition of
Listis incorrect. How would you fix it?template <typename elemType> class ListItem; template <typename elemType> class List { public: List<elemType>(); List<elemType>(const List<elemType> &); List<elemType>& operator=(const List<elemType> &); ~List(); void insert(ListItem *ptr, elemType value); private: ListItem *front, *end; };
use of class template ListItem requires template arguments.
void insert(ListItem<elemType> *ptr, elemType value);
ListItem<elemType> *front, *end;Write your own version of the
BlobandBlobPtrtemplates. including the variousconstmembers that were not shown in the text.
Blob, BlobPtr and ConstBlobPtr | Test
Explain which kind of friendship you chose for the equality and relational operators for
BlobPtr.
General friendship that each instantiation of BlobPtr grants access to the version of the equality and relational operators instantiated with the same type.
Write a
Screenclass template that uses nontype parameters to define the height and width of theScreen.
Implement input and output operators for your
Screentemplate. Which, if any, friends are necessary in classScreento make the input and output operators work? Explain why each friend declaration, if any, was needed.
same reason with Blob.
Rewrite the
StrVecclass (§ 13.5, p. 526) as a template namedVec.
What, if any, are the differences between a type parameter that is declared as a
typenameand one that is declared as aclass? When musttypenamebe used?
When we want to inform the compiler that a name represents a type, we must use the keyword typename, not class.
Explain each of the following function template declarations and identify whether any are illegal. Correct each error that you find.
(a) template <typename T, U, typename V> void f1(T, U, V); (b) template <typename T> T f2(int &T); (c) inline template <typename T> T foo(T, unsigned int*); (d) template <typename T> f4(T, T); (e) typedef char Ctype; template <typename Ctype> Ctype f5(Ctype a);
Fixed:
(a) template <typename T, typename U, typename V> void f1(T, U, V); // identifier 'U'
(b) template <typename T> T f2(int &); // typename would be hidden
(c) template <typename T> inline T foo(T, unsigned int*); // inline must be after template
(d) template <typename T> void f4(T, T); // return type should be provided
(e) typedef char Ctype;
template <typename T> T f5(Ctype a); // the typename hides this typedefWrite a function that takes a reference to a container and prints the elements in that container. Use the container’s
size_typeandsizemembers to control the loop that prints the elements.
Rewrite the function from the previous exercise to use iterators returned from
beginandendto control the loop.
Write your own version of
DebugDelete.
DebugDelete|h | DebugDelete|cpp
Revise your
TextQueryprograms from 12.3 (p. 484) so that theshared_ptrmembers use aDebugDeleteas their deleter (12.1.4, p. 468).
TestQuery|h | TestQuery|cpp | TestQuery|test
Predict when the call operator will be executed in your main query program. If your expectations and what happens differ, be sure you understand why.
when input the q to quit runQueries function. Exercise 16.22's output can check this.
Add a constructor that takes two iterators to your
Blobtemplate.
Explain the meaning of these declarations
extern template class vector<string>; template class vector<Sales_data>;
vector<string> instantiation declaration here, it must be instantiated elsewhere in the program.
vector<Sales_data> instantiates all members of the class template here.
Assuming
NoDefaultis a class that does not have a default constructor, can we explicitly instantiatevector<NoDefault>? If not, why not?
For each labeled statement explain what, if any, instantiations happen. If a template is instantiated, explain why; if not, explain why not.
template <typename T> class Stack { }; void f1(Stack<char>); // (a) class Exercise { Stack<double> &rsd; // (b) Stack<int> si; // (c) }; int main() { Stack<char> *sc; // (d) f1(*sc); // (e) int iObj = sizeof(Stack< string >); // (f) }
Solution:
- (a) No instantiation, compiles, it got instantiated when called.
- (b) No instantiation, compiles, references and pointers doesn't need instantiation
- (c) Instantiation. Doesn't compile!
- (d) No instantiation, compiles, references and pointers doesn't need instantiation
- (e) Instantiation of
Stack<char>. Doesn't compile! - (f) Instantiation of
Stack<std::string>. Doesn't compileNo instantiation, compiles, references and pointers doesn't need instantiation!
Solution from How is a template instantiated? - Stack Overflow
Write your own versions of
shared_ptrandunique_ptr.
shared_ptr | unique_ptr | test
Revise your
Blobclass to use your version ofshared_ptrrather than the library version.
Rerun some of your programs to verify your shared_ptr and revised Blob classes. (Note: Implementing the weak_ptr type is beyond the scope of this Primer, so you will not be able to use the BlobPtr
class with your revised Blob.)
check Exercise 16.28
Explain how the compiler might inline the call to the deleter if we used
DebugDeletewithunique_ptr.
The compiler will set the default deleter type as DebugDelete, which will be executed is known at compile time.
What happens during template argument deduction?
During template argument deduction, the compiler uses types of the arguments in the call to find the template arguments that generate a version of the function that best matches the given call.
Name two type conversions allowed on function arguments involved in template argument deduction.
constconversions- Array- or function-to-pointer conversions
Given only the following code, explain whether each of these calls is legal. If so, what is the type of
T? If not, why not?template <class T> int compare(const T&, const T&); (a) compare("hi", "world"); (b) compare("bye", "dad");
- (a): illegal, as two types are different, the first type being
const char[3],secondconst char[6] - (b): legal, the type is
const char(&)[4].
Which, if any, of the following calls are errors? If the call is legal, what is the type of T? If the call is not legal, what is the problem?
template <typename T> T calc(T, int); template <typename T> T fcn(T, T); double d; float f; char c; (a) calc(c, 'c'); (b) calc(d, f); (c) fcn(c, 'c'); (d) fcn(d, f);
- (a) legal, type is
char. - (b) legal, type is
double. - (c) legal, type is
char. - (d) illegal,
disdouble, butfisfloat, they are totally different type.
What happens in the following calls:
template <typename T> f1(T, T); template <typename T1, typename T2) f2(T1, T2); int i = 0, j = 42, *p1 = &i, *p2 = &j; const int *cp1 = &i, *cp2 = &j; (a) f1(p1, p2); (b) f2(p1, p2); (c) f1(cp1, cp2); (d) f2(cp1, cp2); (e) f1(p1, cp1); (f) f2(p1, cp1);
At first, there are some error in function declarations.
f1,f2should have return type, maybevoid?- In
f2,typename T2)should betypename T2>.
Then, the answers:
- (a)
Tisint* - (b)
T1andT2are bothint* - (c)
Tisconst int* - (d)
T1andT2are bothconst int* - (e) error,
p1isint*,cp1isconst int*, they are different type - (f)
T1isint*,T2isconst int*
The library
maxfunction has two function parameters and returns the larger of its arguments. This function has one template type parameter. Could you callmaxpassing it an int and a double? If so, how? If not, why not?
Yes. Specify the parameter explicitly:
int a = 6; double b = 6.1231; std::cout << std::max<long double>(a, b) << std::endl;Normal conversions also apply for arguments whose template type parameter is explicitly specified.
When we call
make_shared(12.1.1, p. 451), we have to provide an explicit template argument. Explain why that argument is needed and how it is used.
Because when we call make_shared, it is allowed for no argument. Then, we have nothing to deduce the type of the return type.
Use an explicit template argument to make it sensible to pass two string literals to the original version of compare from 16.1.1 (p.652).
std::cout << compare<std::string>("czwp", "czyz") << std::endl;
^^^^^^^^^^^^^Is the following function legal? If not, why not? If it is legal, what, if any, are the restrictions on the argument type(s) that can be passed, and what is the return type?
template <typename It>
auto fcn3(It beg, It end) -> decltype(*beg + 0) {
// process the range
return *beg; // return a copy of an element from the range
}legal. But only type that support this + 0 operation can be passed, and the return type depends on the what type the operator + return.
Write a version of
sumwith a return type that is guaranteed to be large enough to hold the result of the addition.
template <typename T1, typename T2>
auto sum(T1 a, T2 b) -> decltype(a + b) {
return a + b;
}More safer solution: <Better sum>
Determine the type of T and of val in each of the following calls:
template <typename T> void g(T&& val); int i = 0; const int ci = i; (a) g(i); (b) g(ci); (c) g(i * ci);
- (a) T:
int&val:int& &&->int & - (b) T:
const int&val:const int& &&->const int & - (c) T:
intval:int &&
When we pass an lvalue
intto a function parameter that is an rvalue reference to a template type parameterT&&, the compiler deduces the template type parameter as the argument’s lvalue reference typeint &.
X& &,X& &&, andX&& &all collapse to typeX&.
Using the function defined in the previous exercise, what would the template parameter of
gbe if we calledg(i = ci)?
int&
Using the same three calls as in the first exercise, determine the types for
Tifg’s function parameter is declared asT(notT&&). What ifg’s function parameter isconst T&?
Whatever g's function parameter is declared as T or const T&, the T's type in this three case would always int.
Given the following template, explain what happens if we call
gon a literal value such as 42. What if we callgon a variable of typeint?template <typename T> void g(T&& val) { vector<T> v; }
If we call g on a literal value such as 42, T should be int, and we get a tempoary variable v, which type is vector<int>. If we call g on a variable of type int, then val should be a lvalue, T should be int&(because int& && ==> int&), then we would declared a v as vector<int&>. But the component type of vector must be assignable, the references are not assignable, thus, vector<int&> is not allowed, the compiler would complain about it.
Explain this loop from
StrVec::reallocatein 13.5 (p.530):for (size_t i = 0; i != size(); ++i) alloc.construct(dest++, std::move(*elem++));
Since C++11, std::allocator::construct's second parameter is Args&&... args. *elem++ is a certain lvalue, and would be casted to a rvalue reference by std::move, then the construct would call the move constructor of std::string rather than copy constructor.
Write your own version of the flip function and test it by calling functions that have lvalue and rvalue reference parameters.
Explain what happens in each of the following calls:
template <typename T> void f(T); //1
template <typename T> void f(const T*); //2
template <typename T> void g(T); //3
template <typename T> void g(T*); //4
int i = 42, *p = &i;
const int ci = 0, *p2 = &ci;
g(42); g(p); g(ci); g(p2);
f(42); f(p); f(ci); f(p2);Answer:
g(42); // type: int(rvalue) call template 3 T: int instantiation: void g(int)
g(p); // type: int * call template 4 T: int instantiation: void g(int *)
g(ci); // type: const int call template 3 T: const int instantiation: void g(const int)
g(p2); // type: const int * call template 4 T: const int instantiation: void g(const int *)
f(42); // type: int(rvalue) call template 1 T: int instantiation: void f(int)
f(p); // type: int * call template 1 T: int * instantiation: void f(int *)
// f(int *) is an exact match for p(int *) while f(const int *) has a conversion from int * to const int *.
f(ci); // type: const int call template 1 T: const int instantiation: void f(const int)
f(p2); // type: const int * call template 2 T：int instantiation: void f(const int *)Define the functions from the previous exercise so that they print an identifying message. Run the code from that exercise. If the calls behave differently from what you expected, make sure you understand why.
Determine what sizeof...(Args) and sizeof...(rest) return for each call to foo in this section.
template <typename T, typename ... Args>
void foo(const T & t, const Args & ... rest);
int i = 0; double d = 3.14; string s = "how";
foo(i, s, 42, d); // input in Args: string, int(rvalue), double sizeof...(Args): 3 sizeof...(rest): 3
foo(s, 42, "hi"); // input in Args: int(rvalue), const char[3] sizeof...(Args): 2 sizeof...(rest): 2
foo(d, s); // input in Args: string sizeof...(Args): 1 sizeof...(rest): 1
foo("hi"); // input in Args: None sizeof...(Args): 0 sizeof...(rest): 0
foo(i, s, s, d); // input in Args: string, string, double sizeof...(Args): 3 sizeof...(rest): 3Write a program to check your answer to the previous question.