[英]Understanding templates in c++
我正在嘗試運行以下程序,但是會生成編譯錯誤:
#ifndef TEMPLATE_SUM_H_
#define TEMPLATE_SUM_H_
template<typename T>
class sum
{
public:
sum() {
val_1 = 0;
val_2 = 0;
}
sum(T a, T b) {
val_1 = a;
val_2 = b;
}
friend std::ostream& operator<<(std::ostream &, const sum<> &);
private:
T val_1, val_2;
T result() const;
};
#endif
源文件:
include <iostream>
#include "inc/sum.h"
template<typename T>
T sum<T>::result() const {
return (val_1 + val_2);
}
template<typename T>
std::ostream& operator<<(std::ostream& os, const sum<T>& obj) {
//std::ostream& operator<<(std::ostream& os, sum<T>& obj) {
os << obj.result();
return os;
}
int main()
{
sum<int> int_obj(15, 15);
sum<float> float_obj(5.2, 3.5);
std::cout << "result of int = " << int_obj << std::endl;
std::cout << "result of float = " << float_obj << std::endl;
return 0;
}
使用g ++(4.4.3)編譯時,會產生以下錯誤:
In file included from template.cpp:2:
inc/sum.h:18: error: wrong number of template arguments (0, should be 1)
inc/sum.h:5: error: provided for ‘template<class T> class sum’
template.cpp: In function ‘std::ostream& operator<<(std::ostream&, const sum<T>&) [with T = int]’:
template.cpp:20: instantiated from here
template.cpp:5: error: ‘T sum<T>::result() const [with T = int]’ is private
template.cpp:12: error: within this context
template.cpp: In function ‘std::ostream& operator<<(std::ostream&, const sum<T>&) [with T = float]’:
template.cpp:21: instantiated from here
template.cpp:5: error: ‘T sum<T>::result() const [with T = float]’ is private
template.cpp:12: error: within this context
1)有人可以幫助我確定錯誤嗎? 還請提出一些鏈接,在這些鏈接中,我可以找到有關如何在c ++中使用模板的簡要絕對詳細信息。
2)我讀到在頭文件中聲明並單獨定義的模板化func / classs易於鏈接錯誤。 任何人都可以解釋/闡述嗎? 在上面的示例中是否有可能鏈接錯誤?
聲明如下:
“如果在.h文件中聲明了模板或內聯函數,請在同一文件中對其進行定義。這些結構的定義必須包含在使用它們的每個.cpp文件中,否則程序可能無法在某些構建配置中鏈接 ”。
可以使用更簡單的方式完成此示例,而無需使用重載運算符等。但是,我正在嘗試學習/練習模板並嘗試一些功能。
您需要為friend
函數聲明提供單獨的模板定義:
template<typename U>
friend std::ostream& operator<<(std::ostream &, const sum<U> &);
friend
聲明不繼承封閉類的模板參數。
一個簡單的示例源,開始;
計算器
#ifndef CALCULATOR_H
#define CALCULATOR_H
template <class TYPE>
class Calculator{
public:
Calculator();
TYPE Sum(TYPE param1, TYPE param2);
};
/**
* To avoid template related compilation error
* when templates are used in header and source files
*
* This class file has been removed from the project-source file.
* However, is present in the project folder
* Gets compiled with the header-file (being included)
*/
#include "Calculator.cpp"
#endif
Calculator.cpp
#include <iostream>
using namespace std;
#include "Calculator.h"
template <class TYPE>
Calculator<TYPE>::Calculator()
{
}
template <class TYPE>
TYPE Calculator<TYPE>::Sum(TYPE param1, TYPE param2){
cout << "Calculator::sum" << endl;
cout << param1 <<endl;
cout << param2 <<endl;
TYPE result = param1 + param2 ;
return result;
}
Main.cpp
#include <iostream>
using namespace std;
#include "Calculator.h"
int main(int argc, const char * argv[]) {
cout << "Hello, Calculator!\n";
Calculator<int> cObj;
int out = cObj.Sum(2,3);
cout << "out : " << out << endl;
Calculator<string> cObjS;
string outS = cObjS.Sum("A", "B");
cout << "outS : " << outS << endl;
cout << "Bye, Calculator!\n";
return 0;
}
另外,您可以參考post ,以了解如何保留模板源和標頭內容,以及如何解決編譯和鏈接器問題(有原因)。
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