c – 模板类,友元运算符<< overload

发布时间：2020-12-16 09:30:46 所属栏目：百科来源：网络整理

导读：我正试图重载“”模板类的运算符.我在.h文件中定义了类,并在.cpp文件中实现了它. /tmp/ccjJIJhO.o: In function `main':main.cpp:(.text+0xad): undefined reference to `std::basic_istreamchar,std::char_traitschar operatorint(std::basic_istreamchar,s

我正试图重载“<<”模板类的运算符.我在.h文件中定义了类,并在.cpp文件中实现了它.

/tmp/ccjJIJhO.o: In function `main':
main.cpp:(.text+0xad): undefined reference to `std::basic_istream<char,std::char_traits<char> >& operator>><int>(std::basic_istream<char,std::char_traits<char> >&,FeatureVector<int>&)'
main.cpp:(.text+0xba): undefined reference to `std::basic_ostream<char,std::char_traits<char> >& operator<< <int>(std::basic_ostream<char,FeatureVector<int> const&)'
collect2: ld returned 1 exit status

类定义：

COMMON.H

#include <iostream>
using namespace std;

featurevector.h

#ifndef FEATURE_VECTOR_H
#define FEATURE_VECTOR_H

#include <common.h>

template < class FEAT_TYPE >
class FeatureVector;

template < class FEAT_TYPE >
istream & operator >> (istream &,FeatureVector<FEAT_TYPE> &);

template < class FEAT_TYPE >
ostream & operator << (ostream &,const FeatureVector<FEAT_TYPE> &);

template < class FEAT_TYPE >
class FeatureVector{
    public:
        FeatureVector(int = 0);
        ...
        friend istream & operator >> <>(istream &,FeatureVector<FEAT_TYPE> & );
        friend ostream & operator << <>(ostream &,const FeatureVector<FEAT_TYPE> &);
        ...
        ~FeatureVector();

    private:
        int m_nDim;
        FEAT_TYPE * m_pFeat;
};
#endif

featurevector.cpp

#include <featurevector.h>
...
template < class FEAT_TYPE >
istream & operator >> (istream & input,FeatureVector< FEAT_TYPE> & refFeat ){

    int d;

    for(d=0; d < refFeat.getDim(); d++){
        input >> refFeat.m_pFeat[d];
    }

    return (input);
}

template < class FEAT_TYPE >
ostream & operator << (ostream & output,const FeatureVector< FEAT_TYPE > & refFeat ){

    int d;

    for(d=0; d < refFeat.getDim(); d++){
        output << refFeat.m_pFeat[d] << " ";
    }

    output << endl;

    return (output);
}
...
#include "featurevector-impl.cpp"

featurevector-impl.cpp

template class FeatureVector<int>;
//template istream & operator >> <>(istream &,FeatureVector<int> &);
//template ostream & operator << <>(ostream &,const FeatureVector<int> &);

mylib.h

#ifndef MY_LIB_H
#define MY_LIB_H
#include <featurevector.h>
#endif

main.cpp中

#include <mylib.h>
#include <common.h>

int main(){
    FeatureVector<int> pFeat(10);
    cin >> (pFeat);
    cout << (pFeat);

    return (0);
}

Makefile与“mylib”相关联

INC=./inc
SRC=./src
LIB=./lib
OBJ=./obj

CC=g++
CFLAGS=-O3 -Wall

mylib: $(LIB)/mylib.a
echo "mylib was created!..."

$(LIB)/mylib.a: 
$(OBJ)/featurevector.o 
    ar csr $(LIB)/mylib.a 
$(OBJ)/featurevector.o 

$(OBJ)/featurevector.o: $(SRC)/featurevector.cpp
    $(CC) -c $(CFLAGS) $(SRC)/featurevector.cpp -I$(INC)  
    -o $(OBJ)/featurevector.o

clean:
    rm -rf $(LIB)/*.a
    rm -rf $(OBJ)/*.o

main.cpp的Makefile(带有Makefile的main.cpp位于“app”目录下)

LIB=../lib
INC=../inc
OBJ=../obj
BIN=../bin

CC=g++
CFLAGS=-O3 -Wall
LFLAGS=-lmylib -lm

$@.cpp: $(LIB)/mylib.a $@.cpp
    cd ..; make; cd app;
$(CC) $(CFLAGS) $@.cpp -o $(BIN)/$@ -I$(INC) -L$(LIB) $(LFLAGS)

clean:
    rm -rf $(BIN)/*

解决方法

根据 this,您必须在类定义中将该函数称为模板.

class.h

#include <iostream>
using std::ostream;

template <typename T>
class A {
  public:
    ...

    template <typename J> // <-- CAUTION!
    friend ostream &operator<<(ostream &output,const A<J> &a);
};

class.cpp

#include "class.h"
...
template <typename T>
ostream &operator<<(ostream &output,const A<T> &a) {
  // Your implementation
  return output;
}

...
template ostream &operator<<(ostream &output,const A<int> &a);
template ostream &operator<<(ostream &output,const A<float> &a);

如果行模板< typename J>被删除,编译错误“欠定参考”来了.

（编辑：李大同）

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c – 模板类,友元运算符&lt;&lt; overload

解决方法

c – 模板类,友元运算符<< overload