[英]templated class can't redefine operator[]
I've this class 我上这堂课
namespace baseUtils {
template<typename AT>
class growVector {
int size;
AT **arr;
AT* defaultVal;
public:
growVector(int size, AT* defaultVal ); //Expects number of elements (5) and default value (NULL)
AT*& operator[](unsigned pos);
int length();
void reset(int pos); //Resets an element to default value
void reset(); //Resets all elements to default value
~growVector();
};
}
and this is the implementation for operator[] 这是operator []的实现
template<typename AT>
AT*& growVector<AT>::operator [](unsigned pos){
if (pos >= size){
int newSize = size*2;
AT** newArr = new AT*[newSize];
memcpy(newArr, arr, sizeof(AT)*size);
for (int i = size; i<newSize; i++)
newArr[i] = defaultVal;
size = newSize;
delete arr;
arr = newArr;
}
return arr[pos];
}
(yes I do realize i don't check if size*2 >= pos... but that's not the point now) if I use it in code like: (是的,我的确意识到我不检查size * 2> = pos ...但这不是重点)如果我在代码中使用它,例如:
int main() {
growVector<char> gv();
char* x = NULL;
for (int i = 0; i< 50; i++){
gv[i] = x;
}
gv.reset();
return 0;
}
the compiler says 编译器说
../src/base.cpp:98: warning: pointer to a function used in arithmetic
../src/base.cpp:98: error: assignment of read-only location ‘*(gv + ((unsigned int)i))’
../src/base.cpp:98: error: cannot convert ‘char*’ to ‘baseUtils::growVector<char>()’ in assignment
referring to the line gv[i] = x; 参照线gv [i] = x; (seems like it doesn't see the redefinition of [])
(似乎没有看到[]的重新定义)
Why???? 为什么???? What am I missing?
我想念什么?
After correcting the constructor problem I've the linker sayng: 更正构造函数问题后,我有了链接器sayng:
/home/dario/workspace/base/Debug/../src/base.cpp:95: undefined reference to `baseUtils::growVector<char>::growVector(int, char*)'
/home/dario/workspace/base/Debug/../src/base.cpp:98: undefined reference to `baseUtils::growVector<char>::operator[](unsigned int)'
/home/dario/workspace/base/Debug/../src/base.cpp:100: undefined reference to `baseUtils::growVector<char>::reset()'
/home/dario/workspace/base/Debug/../src/base.cpp:101: undefined reference to `baseUtils::growVector<char>::~growVector()'
/home/dario/workspace/base/Debug/../src/base.cpp:101: undefined reference to `baseUtils::growVector<char>::~growVector()'
like it cannot link... why??? 像它无法链接...为什么? :O
:O
The problem is your declaration 问题是你的声明
growVector<char> gv();
The compiler interprets this as declaring a function called gv
which returns a growVector<char>
, not as an object as you indend. 编译器将此解释为声明了一个名为
gv
的函数,该函数返回growVector<char>
,而不是您所追求的对象。 Since there isn't a default constructor, this wouldn't compile anyway. 由于没有默认的构造函数,因此无论如何都不会编译。 Change it to:
更改为:
growVector<char> gv(0,0);
The compiler thinks this line 编译器认为这一行
growVector<char> gv();
is declaring a function, rather than a variable. 在声明一个函数,而不是一个变量。 Drop the
()
and things should work. 删除
()
,一切正常。
I just would like to point out that it is a good practice to have two versions of subscript [] operator in the class: const (which will be used for r-value) and non-const. 我只想指出,在类中有两个版本的下标[]运算符是一种好习惯:const(将用于r值)和non-const。 You have implemented non-const version but it can not be used in const functions or in any function that receive instance of your class as const reference or pointer pointer to const.
您已经实现了非const版本,但不能在const函数或任何将类实例作为const引用或指向const的指针的类中使用。
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