Templates in C++ - GeeksforGeekshttps://www.geeksforgeeks.org/templates-cpp/
What is templateA template is a simple and yet very powerful tool in C++. The simple idea is to pass data type as a parameter so that we don’t need to write the same code for different data types. For example, a software company may need sort() for different data types. Rather than writing and maintaining the multiple codes, we can write one sort() and pass data type as a parameter.
C++为支持template引入的关键字C++ adds two new keywords to support templates: ‘template’ and ‘typename’. The second keyword can always be replaced by keyword ‘class’.
How do templates work?Templates are expanded at compiler time. This is like macros. The difference is, the compiler does type checking before template expansion. The idea is simple, source code contains only function/class, but compiled code may contain multiple copies of same function/class.
编译时展开多个，这也是常说的使用STL代码会变大
怎样看到展开后的代码样子？Templates are part of the language, not some pre-processor pass - they are processed by the compiler, just like other code. (并不是像C一样预处理展开宏之后的代码)
但是可以通过
➜ c++ nm a.out | c++filt| grep myMax (证明生成了多个函数)
000000000000134f W char myMax
0000000000001323 W double myMax
0000000000001303 W int myMax
g++ -fdump-rtl-all learn_c++.cc 会生成多个文件看 以dfinish结尾的文件
其中有
;; Function myMax
;; Function myMax
;; Function myMax
虽然不能通过编译的中间文件看到 template 展开后的样子，但是通过上面的方式看到展开成多个函数是无疑的。
Function TemplatesWe write a generic function that can be used for different data types.
#include
using namespace std;
// One function works for all data types. This would work
// even for user defined types if operator '>' is overloaded
template
T myMax(T x, T y)
{
return (x > y)? x: y;
}
int main()
{
cout << myMax
cout << myMax
cout << myMax
return 0;
}
Class Templates Like function templates, class templates are useful when a class defines something that is independent of the data type. Can be useful for classes like linkedList, BinaryTree, Stack, Queue, Array, etc.
Following is a simple example of template Array class.
#include
using namespace std;
template
class Array {
private:
T *ptr;
int size;
public:
Array(T arr[], int s);
void print();
};
template
Array
ptr = new T[s];
size = s;
for(int i = 0; i < size; i++)
ptr[i] = arr[i];
}
template
void Array
for (int i = 0; i < size; i++)
cout<<" "<<*(ptr + i);
cout<
int main() {
int arr[5] = {1, 2, 3, 4, 5};
Array
a.print();
return 0;
}
Yes, like normal parameters, we can pass more than one data types as arguments to templates. The following example demonstrates the same.
Yes, like normal parameters, we can specify default arguments to templates. The following example demonstrates the same.
Both function overloading and templates are examples of polymorphism feature of OOP. Function overloading is used when multiple functions do similar operations, templates are used when multiple functions do identical operations.
What happens when there is a static member in a template class/function?
Each instance of a template contains its own static variable.
What is template specialization?
Template specialization allows us to have different code for a particular data type.
Can we pass nontype parameters to templates?
We can pass non-type arguments to templates. Non-type parameters are mainly used for specifying max or min values or any other constant value for a particular instance of a template. The important thing to note about non-type parameters is, they must be const. The compiler must know the value of non-type parameters at compile time. Because the compiler needs to create functions/classes for a specified non-type value at compile time. In below program, if we replace 10000 or 25 with a variable, we get a compiler error.
