C++类类型的函数报错

我自定义了一个twoTuple类,在main函数里想要定义一个twoTuple类型的函数,但是报错显示缺少;
class twoTuple
{
public:
    twoTuple(string type, string value);
    ~twoTuple();

    string type;    //类型
    string value;    //值

    void setType(string type) {    this->type = type; }

    string getType() { return type; }

    void setValue(string value) { this->value = value; }

    string getValue() {    return value; }

    //返回二元式字符串
    string show() {
        return "(" + type + "," + value + ")";
    }

private:

};

twoTuple::twoTuple(string type, string value)
{
    this->type = type;
    this->value = value;
}

twoTuple::~twoTuple()
{
}


int main()
{
  twoTuple getNext(){

}
  return 0;
}

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函数内不能嵌套函数,把这个函数放到main函数上面