Instantiation of Class-Template Member Functions

[!quote]
By default, a member function of a class template is instantiated only if the program uses that member function.

由 Blob<std::string> {} 实例化的类：

template<>
class Blob<std::basic_string<char> >
{
  
  public: 
  std::basic_string<char> & back();
  
  inline std::basic_string<char> & operator[](size_t i);
  
  inline void pop_back();
  
  Blob();
  
  Blob(std::initializer_list<std::basic_string<char> > il)
  : data{std::make_shared<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > >(il)}
  {
  }
  
  
  private: 
  void check(size_t i, const std::basic_string<char> & msg) const;
  
  std::shared_ptr<std::vector<std::basic_string<char>, std::allocator<std::basic_string<char> > > > data;
  public: 
  // inline ~Blob() noexcept = default;
};

没有使用的函数只有声明没有具体的定义。
对不使用的函数不进行实例化的好处：

[!quote]
let us instantiate a class with a type that may not meet the requirements for some of the template's operations.

可以根据模板类型调用指定函数