是否可以强制新模板实例化?

时间:2013-09-17 13:40:31

标签: c++ templates

我希望得到类似以下(psuedo-C ++)类的内容。基本思路是将任何新值初始化为最后一个设定值。

StickyData.hpp

template<typename T>
class StickyData {
  static T s_lastValue;
  T m_value;
  StickyData() { m_value = s_lastValue; } // Initialize to last set value
  T& operator = (T& rhs)  { 
       m_value = s_lastValue = rhs;
  }
}

template<> StickyData<T>::s_lastValue;

UseData.cpp

typedef int Ford;
typedef int GM;
typedef int Toyota;

StickyData<Ford> aFord;
StickyData<GM> aGM;
StickyData<Toyota> aToyota;

aFord = 10;
aGM = 2
aToyota = 20;

StickyData<Ford> anotherFord; // I want this automatically initialized to 10

为了实现这一点,我需要为这些typedef中的每一个都使用StickyData进行独特的编译。我实际上认为这将按原样运行,但是当我运行此代码时,我只为所有这些不同的模板实例创建了1个静态int。

我是否可以针对相同的基础值类型以某种方式强制执行新模板编译?

1 个答案:

答案 0 :(得分:2)

在您的示例代码中,Fords,GM和Toyotas都是相同的类型。您应该将它们声明为实际的单独类型:

struct Toyota {};
struct Ford   {};
struct GM     {};

目前,您的代码与以下内容相同:

HoldData<int> aFord;
HoldData<int> aGM;
HoldData<int> aToyota;

以上是我上面建议的完整实现:​​

#include <iostream>

template<typename Hash, typename Value>
struct StickyData {
        static Value s_lastValue;
        Value m_value;
        StickyData(): m_value(s_lastValue) {}

        StickyData<Hash, Value> & operator=(Value const & rhs) {
                m_value = s_lastValue = rhs;
                return *this;
        }
};

struct GM {};
struct Ford {};
struct Honda {};

template <typename Hash, typename Value> Value StickyData<Hash, Value>::s_lastValue = 0;

int main() {
        StickyData<GM, int> aGM;
        StickyData<Ford, int> aFord;
        StickyData<Honda, double> aHonda;

        aGM = 3;
        aFord = 4;
        aHonda = 7.89;

        std::cout << "aGM:    " << aGM.m_value << "\n";
        std::cout << "aFord:  " << aFord.m_value << "\n";
        std::cout << "aHonda: " << aHonda.m_value << "\n";
        std::cout << "\n";

        StickyData<GM, int> aNewGM;
        std::cout << "aNewGM: " << aGM.m_value << "\n";
}

将输出:

aGM:    3
aFord:  4
aHonda: 7.89

aNewGM: 3

回应评论: “...但编译器确实知道差异,例如,如果你有typedef'ed参数,你可以标记一些明确的东西然后不接受类型'int'作为参数......“

我有以下代码:

typedef int foo;

class C {
    explicit C(int x);
    explicit C(foo x);
};

此代码无法编译,并出现以下错误:

blah.cc:5:11: error: ‘C::C(foo)’ cannot be overloaded
  explicit C(foo x);
           ^
blah.cc:4:11: error: with ‘C::C(int)’
  explicit C(int x);
           ^