Basic Template Terminology
1. "Class Template" or "Template Class"?
class template: the class is a template.
template class: a synonym(同義詞) for class template; to refer to classes generated from templates(由模板產生(實例化)的類). to refer to classes with a name that is a template-id.
Because of this imprecision, template class is avoided in this book.
2. Instantiation and specialzation
Instantiation: 實例化. The process of creating a regular class, function, or member function from a template by substituting actual values for its arguments is called template instantiation.
Specialzation:特例，在Chapter 3中有描述
3. Declarations vs Definitions
declaration is a C++ construct that introduces or reintroduceds a name into? a C++ scope.
class?C;?//?a?declaration?of?C?as?a?class
void?f(int?p);?//?a?declaration?of?f()?as?a?function?and?p?as?a?named?parameter
extern?int?v;?//?a?declaration?of?v?as?a?variable
Declarations become definitions when the details of their structure are made known or, in the case of variables, when storage space must be allocated.
class?C
{};?//?definition(and?declaration)?of?class?C
void?f(int?p){?//?defini
???std::cout<<?p?<<?std::endl;
}
extern?int?v?=?1;??//?an?initializer?makes?this?a?definition?for?v
int?w;???//?global?variable?declaration?not?preceded?by?extern?are?also?definitionsBy extension, the declaration of?a class template or function template is called a definition if it has a body, Hence,
template?<typename?T>
void?func(T);
is a declaration that is not a definition, whereas
template?<typename?T>
class?S{};
is in fact a definition
Using Templates in Practice
1. Templates challenge the classic compiler-plus-linker model. Therefore there are different approaches to organize template code: the inclusion model, explicit instantiation, and the separation model
Most C and C++ programmers organize their nontemplate code largely as follows:
1) Classes and other types are entirely placed in header files.
2) For global variables and (noninline) functions, only a declaration is put in a header file, and the definition goes into a so-called dot-C file.
The works well: It makes the needed type definition easily available throughout the program and avoids duplicate definition errors on variables and functions from the linker.
With the convention in mind, we declare the template in a header file:
#ifndef?MYFIRST_HPP
#define?MYFIRST_HPP
//?declaration?of?template
template?<typename?T>?
void?print_typeof?(T?const&);
#endif?//?MYFIRST_HPP The implementation of the function is placed in a dot-C file:
#include?<iostream>
#include?<typeinfo>
#include?"myfirst.hpp"
//?implementation/definition?of?template
template?<typename?T>
void?print_typeof?(T?const&?x)
{
????std::cout?<<?typeid(x).name()?<<?std::endl;
} Finally, we use the template in another dot-C file, into which our template declaration is #include:
#include?"myfirst.hpp"
//?use?of?the?template
int?main()
{
????double?ice?=?3.0;
????print_typeof(ice);??//?call?function?template?for?type?double
} A C++ compiler will most likely accept this program without any problems,but
the linker will probably report an error,
implying that there is no definition of the function print_typeof().
In order for a template to be? instantiated, the compiler
must know which definition should be instantiated and for what template arguments it should be instantiated. Unfortunately, these
two pieces of information are in files that are compiled separatedly. Therefore, when our compiler sees the call to print_typeof() but has no definition in sight to instantiate this function for
double, it just assumes that such a definition is provided elsewhere and creates a reference(
for the linker to resolve,linker) to that definition. On the other hand, when the compiler processes the file myfirst.cpp, it has no indication at that point that it must instantiate the template definition it contains for specific arguments.
The Inclusion Model
Rewrite the header-file, including template definition.
#ifndef?MYFIRST_HPP
#define?MYFIRST_HPP
#include?<iostream>
#include?<typeinfo>
//?declaration?of?template
template?<typename?T>?
void?print_typeof?(T?const&);
//?implementation/definition?of?template
template?<typename?T>
void?print_typeof?(T?const&?x)
{
????std::cout?<<?typeid(x).name()?<<?std::endl;
}
#endif?//?MYFIRST_HPP disadvantage: the cost is not the result of size of the template definition itself, but the result of the fact that we must also include the header used by the definition of our template-in the case <iostream> and <typeinfo>.\
Explicit Instantiation
To avoid above linker error we can add the following file to our program:
#include?"myfirst.cpp"
//?explicitly?instantiate?print_typeof()?for?type?double
template?void?print_typeof<double>(double?const&); disadvantage: We must carefully keep track of which entities to instantiate. For large projects this quickly becomes an excessive burden.
advantage:
the instantiation can be tuned to the needs of the program.
The overhead of large header is avoided.
The source code of template definition can be kept hidden, but then no additional instantiations can be created by a client program.
Finally, for some applications it can be useful to control the exact location(that is, the object file)of a template instance.
Separation model
2. Usually you should use the inclusion model
3. By separating template code into different header files for declarations and definitions, you can more easily switch between the inclusion model and explicit instantiation
4. The C++ standard defines a separate compilation model for templates(using the keyword export). It is not yet widely available, however.
5. To take advantage of precompiled headers, be sure to keep the same order for #include directives.
6. Debuggin code with templates can be challenging
7. Template instances may have very long names
殺毒就用卡巴：（不敢恭維，原來用過，很耗資源）
木馬就用Ewido：（剛裝上，破解比較麻煩，文件中有整個包，看到大家的評價不錯，官方下載：
http://download.ewido.net/ewido-setup.exe；）
防火墻就用ZoneAlarm：? (還是不用的好，這個好麻煩，總要你確認是否允許聯網，就用xp的防火墻吧)
這三個合起來用，可以使得系統堅若磐石
它們每個都是當今世界最強的，不用是你的損失！！
卡巴和ZoneAlarm同時安裝時，記得在卡巴的實時保護設置里，把那個禁止網絡攻擊保護打上鉤?。。。。?！不然會沖突?。?！ 切記！?。。ㄈ绻话惭b卡巴的話就不用這樣做了）
今天晚上，已經是我兩周以來第二次恢復筆記本的系統了，好在有true image??！。原因都是一樣：在上網找一些軟件時，不幸中招。
我還一直很小心不斷升級Symantec。將防火墻開啟。也真不知道哪個防毒軟件真的好使，也不知道怎么這么多人希望弄這些病毒！
無奈！
摘要: 1. To access a type name that depends on a template parameter, you have to?qualify(?修改，修飾) the name with a leading typename//?print?elements?of?an?STL?containertemplate?<typename?T>void?printcol...
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Chapter 4 Nontype Template Parameters
1. Templates can have template parameters that are values rather than types
With this class, user of the stack could specify this size of the array as the maximum size needed for stack elements
template?<typename?T,?int?MAXSIZE>
class?Stack?{
??private:
????T?elems[MAXSIZE];????????//?elements
????int?numElems;????????????//?current?number?of?elements
??public:
????Stack();??????????????????//?constructor
????void?push(T?const&);??????//?push?element
????void?pop();???????????????//?pop?element
????T?top()?const;????????????//?return?top?element
????bool?empty()?const?{??????//?return?whether?the?stack?is?empty
????????return?numElems?==?0;
????}
????bool?full()?const?{???????//?return?whether?the?stack?is?full
????????return?numElems?==?MAXSIZE;
????}
};
//?constructor
template?<typename?T,?int?MAXSIZE>
Stack<T,MAXSIZE>::Stack?()
??:?numElems(0)???????????????//?start?with?no?elements
{
????//?nothing?else?to?do
}
template?<typename?T,?int?MAXSIZE>
void?Stack<T,MAXSIZE>::push?(T?const&?elem)
{
????if?(numElems?==?MAXSIZE)?{
????????throw?std::out_of_range("Stack<>::push():?stack?is?full");
????}
????elems[numElems]?=?elem;???//?append?element
????++numElems;???????????????//?increment?number?of?elements
}
template<typename?T,?int?MAXSIZE>
void?Stack<T,MAXSIZE>::pop?()
{
????if?(numElems?<=?0)?{
????????throw?std::out_of_range("Stack<>::pop():?empty?stack");
????}
????--numElems;???????????????//?decrement?number?of?elements
}
template?<typename?T,?int?MAXSIZE>
T?Stack<T,MAXSIZE>::top?()?const
{
????if?(numElems?<=?0)?{
????????throw?std::out_of_range("Stack<>::top():?empty?stack");
????}
????return?elems[numElems-1];??//?return?last?element
} 2. You can also define nontype parameters for function templates.
template?<typename?T,?int?VAL>
T?addValue?(T?const&?x)
{
????return?x?+?VAL;
} However, according to the current standard, sets of overloaded functions cannot? be used for template parameter deduction. Thus, you have to cast to the exact type of the function template argument:
std::transform(source.begin(),source.end(),//?start?and?end?of?source
???????????????dest.begin(),//?start?of?destination
???????????????(int(*)(int?const&))addValue<int,5>);//operation
There is a proposal for the standard to fix this behavior so that the cast isn't necessary.
2. You cannot use floating-point numbers, class-type objects, and objects with internal linkage(such as string literals) as arguments for nontype template parameters
這段時間在通過"C++Templates The Complete Guide"這本書學習Templates。發現這本書確實不錯：語言簡明，內容翔實。特別是每章后面的Summery總結得很好。這份讀書筆記就已這個Summery為基礎寫的。
書前面的Prefece和Chapter 1就跳過了。既然來學習Templates就已經知道它的重要性了。
??????????????????????????????? Chapter 2 FunctionTemplates
1. Function templates define a family of functions for different template arguments;
template?
<
typename?T
>
inline?T?
const
&
?max?(T?
const
&
?a,?T?
const
&
?b)
{
????
//
?if?a?<?b?then?use?b?else?use?a
????
return
??a?
<
?b?
?
?b?:?a;
}
2. When you pass template arguments, function templates are instantiated for these argument types.
The process of replacing templates parameters by concrete types is called instantiatin.(at compiled time)
3. You can explicitly qualify the template parameters
{
//
?because?no?automatic?type?conversion?if?allowed?in?templates,so
max(static_cast
<
double
>
(
4
),
4.2
)
//
cast?the?arguments?so?that?they?both?match
max
<
double
>
(
4
,
4.2
)
//
?specify?explicitly?the?type?of?T
}
4. You can overload funciton templates
inline?
int
?
const
&
?max?(
int
?
const
&
?a,?
int
?
const
&
?b)?
{
????
return
??a?
<
?b?
?
?b?:?a;
}
//
?maximum?of?two?values?of?any?type
template?
<
typename?T
>
inline?T?
const
&
?max?(T?
const
&
?a,?T?
const
&
?b)
{
????
return
??a?
<
?b?
?
?b?:?a;
}
//
?maximum?of?three?values?of?any?type
template?
<
typename?T
>
inline?T?
const
&
?max?(T?
const
&
?a,?T?
const
&
?b,?T?
const
&
?c)
{
????
return
?::max?(::max(a,b),?c);
}
int
?main()
{
????::max(
7
,?
42
,?
68
);?????
//
?calls?the?template?for?three?arguments
????::max(
7.0
,?
42.0
);?????
//
?calls?max<double>?(by?argument?deduction)
????::max(
'
a
'
,?
'
b
'
);??????
//
?calls?max<char>?(by?argument?deduction)
????::max(
7
,?
42
);?????????
//
?calls?the?nontemplate?for?two?ints
????::max
<>
(
7
,?
42
);???????
//
?calls?max<int>?(by?argument?deduction)
????::max
<
double
>
(
7
,?
42
);?
//
?calls?max<double>?(no?argument?deduction)
????::max(
'
a
'
,?
42.7
);?????
//
?calls?the?nontemplate?for?two?ints
}
ps: the overload resolution process normally prefers this nontemplate over one generated from the template. the fourth call falls under this rule.
5. When you overload function templates, limit your changes to specifying template parameters explicitly
{
max
<>
(
7
,
42
);
//
call?max<int>?(by?argument?deduction)
}
6. Make sure you see all overloaded versions of funciton templates before you call them
template?
<
typename?T
>
inline?T?
const
&
?max?(T?
const
&
?a,?T?
const
&
?b)
{
????
return
??a?
<
?b?
?
?b?:?a;
}
//
?maximum?of?three?values?of?any?type
template?
<
typename?T
>
inline?T?
const
&
?max?(T?
const
&
?a,?T?
const
&
?b,?T?
const
&
?c)
{
????
return
?max?(max(a,b),?c);??
//
?uses?the?template?version?even?for?ints
}
??????????????????????????????
//
?because?the?following?declaration?comes
???????????????????????????????
//
?too?late:
//
?maximum?of?two?int?values
inline?
int
?
const
&
?max?(
int
?
const
&
?a,?
int
?
const
&
?b)?
{
????
return
??a?
<
?b?
?
?b?:?a;
}
?
? 昨天晚上建立了這個博客。主要想把關于自己學習C++的知識、人生的感悟、還有從網上看到的好東西等等都放在這里。但建立的時候為博客的名字，我和老婆絞盡腦汁。想的名字不是太俗，就是太怪。于是我們就想到問問我兩歲半的兒子，正處于睡意懵懂中的兒子大聲說：“寧帥！”。哈哈。這個是兒子給自己起的名字。好就叫這個了。至于“健康、快樂、勇敢”是我們對他的希望。