學過C++的人，最不陌生的函數就是cout了
那麼，今天，我們就來扒一扒cout背後的故事~
什麼叫cout? ——來自C++新新新新手
cout，說白了就是C++標準輸出函數，例如，你看了很多大佬 的文章，可是，最後的最後，你一臉懵逼
cout啥意思啊？
那什麼叫做標準輸出函數呢？
比如說，你有一天心血來潮，打開了編譯器，單擊了新檔案，卻不知道怎麼繼續編
在CSDN上查詢，道理自己都聽不懂
所以，這就是我寫這篇文章的初衷——幫助C++初學者更加瞭解cout函數
例子1.1 輸出Hello World!
#include <iostream> //標準輸出庫，cout就裝在裏面，這裏，我們把它給取出來，所以要加這個東西
using namespace std;//名稱空間，初學建議不要管他
int main()//main主函數，沒有這個函數程式就無法編譯
{
cout<<"Hello World!";//C++的標準輸出函數
return 0;//return的意思就是執行完語句，計算機給你一個數據，不僅僅可以是0，還可以是-123,12842等等
}
ps:上文的註解已經很詳細了，如果還不懂……建議報班聽聽
所以，cout不僅可以輸出字串，還可以輸出計算的結果哦
例子1.2 1+2等於幾？
#include <iostream>
using namespace std;
int main()
{
cout<<1+2;//輸出1+2等於幾
return 0;
}
哈哈，是不是以後寫作業不用愁啦？
事實證明，下面 下麪幾種寫法也可以
cout<<5*6;
cout<<14/2;
cout<<(5+3)/4;
cout<<"1+2的值是"<<1+2;
怎麼樣，是不是感覺發現了新大陸呢
別急別急，我們還沒把cout的函數原型給大家看呢~
//iostream
extern ostream cout; /// Linked to standard output
//ostream
class basic_ostream : virtual public basic_ios<_CharT, _Traits>
{
public:
// Types (inherited from basic_ios):
typedef _CharT char_type;
typedef typename _Traits::int_type int_type;
typedef typename _Traits::pos_type pos_type;
typedef typename _Traits::off_type off_type;
typedef _Traits traits_type;
// Non-standard Types:
typedef basic_streambuf<_CharT, _Traits> __streambuf_type;
typedef basic_ios<_CharT, _Traits> __ios_type;
typedef basic_ostream<_CharT, _Traits> __ostream_type;
typedef num_put<_CharT, ostreambuf_iterator<_CharT, _Traits> >
__num_put_type;
typedef ctype<_CharT> __ctype_type;
/**
* @brief Base constructor.
*
* This ctor is almost never called by the user directly, rather from
* derived classes' initialization lists, which pass a pointer to
* their own stream buffer.
*/
explicit
basic_ostream(__streambuf_type* __sb)
{ this->init(__sb); }
/**
* @brief Base destructor.
*
* This does very little apart from providing a virtual base dtor.
*/
virtual
~basic_ostream() { }
/// Safe prefix/suffix operations.
class sentry;
friend class sentry;
//@{
/**
* @brief Interface for manipulators.
*
* Manipulators such as @c std::endl and @c std::hex use these
* functions in constructs like "std::cout << std::endl". For more
* information, see the iomanip header.
*/
__ostream_type&
operator<<(__ostream_type& (*__pf)(__ostream_type&))
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// DR 60. What is a formatted input function?
// The inserters for manipulators are *not* formatted output functions.
return __pf(*this);
}
__ostream_type&
operator<<(__ios_type& (*__pf)(__ios_type&))
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// DR 60. What is a formatted input function?
// The inserters for manipulators are *not* formatted output functions.
__pf(*this);
return *this;
}
__ostream_type&
operator<<(ios_base& (*__pf) (ios_base&))
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// DR 60. What is a formatted input function?
// The inserters for manipulators are *not* formatted output functions.
__pf(*this);
return *this;
}
//@}
//@{
/**
* @name Inserters
*
* All the @c operator<< functions (aka <em>formatted output
* functions</em>) have some common behavior. Each starts by
* constructing a temporary object of type std::basic_ostream::sentry.
* This can have several effects, concluding with the setting of a
* status flag; see the sentry documentation for more.
*
* If the sentry status is good, the function tries to generate
* whatever data is appropriate for the type of the argument.
*
* If an exception is thrown during insertion, ios_base::badbit
* will be turned on in the stream's error state without causing an
* ios_base::failure to be thrown. The original exception will then
* be rethrown.
*/
//@{
/**
* @brief Integer arithmetic inserters
* @param __n A variable of builtin integral type.
* @return @c *this if successful
*
* These functions use the stream's current locale (specifically, the
* @c num_get facet) to perform numeric formatting.
*/
__ostream_type&
operator<<(long __n)
{ return _M_insert(__n); }
__ostream_type&
operator<<(unsigned long __n)
{ return _M_insert(__n); }
__ostream_type&
operator<<(bool __n)
{ return _M_insert(__n); }
__ostream_type&
operator<<(short __n);
__ostream_type&
operator<<(unsigned short __n)
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 117. basic_ostream uses nonexistent num_put member functions.
return _M_insert(static_cast<unsigned long>(__n));
}
__ostream_type&
operator<<(int __n);
__ostream_type&
operator<<(unsigned int __n)
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 117. basic_ostream uses nonexistent num_put member functions.
return _M_insert(static_cast<unsigned long>(__n));
}
#ifdef _GLIBCXX_USE_LONG_LONG
__ostream_type&
operator<<(long long __n)
{ return _M_insert(__n); }
__ostream_type&
operator<<(unsigned long long __n)
{ return _M_insert(__n); }
#endif
//@}
//@{
/**
* @brief Floating point arithmetic inserters
* @param __f A variable of builtin floating point type.
* @return @c *this if successful
*
* These functions use the stream's current locale (specifically, the
* @c num_get facet) to perform numeric formatting.
*/
__ostream_type&
operator<<(double __f)
{ return _M_insert(__f); }
__ostream_type&
operator<<(float __f)
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 117. basic_ostream uses nonexistent num_put member functions.
return _M_insert(static_cast<double>(__f));
}
__ostream_type&
operator<<(long double __f)
{ return _M_insert(__f); }
//@}
/**
* @brief Pointer arithmetic inserters
* @param __p A variable of pointer type.
* @return @c *this if successful
*
* These functions use the stream's current locale (specifically, the
* @c num_get facet) to perform numeric formatting.
*/
__ostream_type&
operator<<(const void* __p)
{ return _M_insert(__p); }
/**
* @brief Extracting from another streambuf.
* @param __sb A pointer to a streambuf
*
* This function behaves like one of the basic arithmetic extractors,
* in that it also constructs a sentry object and has the same error
* handling behavior.
*
* If @p __sb is NULL, the stream will set failbit in its error state.
*
* Characters are extracted from @p __sb and inserted into @c *this
* until one of the following occurs:
*
* - the input stream reaches end-of-file,
* - insertion into the output sequence fails (in this case, the
* character that would have been inserted is not extracted), or
* - an exception occurs while getting a character from @p __sb, which
* sets failbit in the error state
*
* If the function inserts no characters, failbit is set.
*/
__ostream_type&
operator<<(__streambuf_type* __sb);
//@}
//@{
/**
* @name Unformatted Output Functions
*
* All the unformatted output functions have some common behavior.
* Each starts by constructing a temporary object of type
* std::basic_ostream::sentry. This has several effects, concluding
* with the setting of a status flag; see the sentry documentation
* for more.
*
* If the sentry status is good, the function tries to generate
* whatever data is appropriate for the type of the argument.
*
* If an exception is thrown during insertion, ios_base::badbit
* will be turned on in the stream's error state. If badbit is on in
* the stream's exceptions mask, the exception will be rethrown
* without completing its actions.
*/
/**
* @brief Simple insertion.
* @param __c The character to insert.
* @return *this
*
* Tries to insert @p __c.
*
* @note This function is not overloaded on signed char and
* unsigned char.
*/
__ostream_type&
put(char_type __c);
/**
* @brief Core write functionality, without sentry.
* @param __s The array to insert.
* @param __n Maximum number of characters to insert.
*/
void
_M_write(const char_type* __s, streamsize __n)
{
const streamsize __put = this->rdbuf()->sputn(__s, __n);
if (__put != __n)
this->setstate(ios_base::badbit);
}
/**
* @brief Character string insertion.
* @param __s The array to insert.
* @param __n Maximum number of characters to insert.
* @return *this
*
* Characters are copied from @p __s and inserted into the stream until
* one of the following happens:
*
* - @p __n characters are inserted
* - inserting into the output sequence fails (in this case, badbit
* will be set in the stream's error state)
*
* @note This function is not overloaded on signed char and
* unsigned char.
*/
__ostream_type&
write(const char_type* __s, streamsize __n);
//@}
/**
* @brief Synchronizing the stream buffer.
* @return *this
*
* If @c rdbuf() is a null pointer, changes nothing.
*
* Otherwise, calls @c rdbuf()->pubsync(), and if that returns -1,
* sets badbit.
*/
__ostream_type&
flush();
/**
* @brief Getting the current write position.
* @return A file position object.
*
* If @c fail() is not false, returns @c pos_type(-1) to indicate
* failure. Otherwise returns @c rdbuf()->pubseekoff(0,cur,out).
*/
pos_type
tellp();
/**
* @brief Changing the current write position.
* @param __pos A file position object.
* @return *this
*
* If @c fail() is not true, calls @c rdbuf()->pubseekpos(pos). If
* that function fails, sets failbit.
*/
__ostream_type&
seekp(pos_type);
/**
* @brief Changing the current write position.
* @param __off A file offset object.
* @param __dir The direction in which to seek.
* @return *this
*
* If @c fail() is not true, calls @c rdbuf()->pubseekoff(off,dir).
* If that function fails, sets failbit.
*/
__ostream_type&
seekp(off_type, ios_base::seekdir);
protected:
basic_ostream()
{ this->init(0); }
template<typename _ValueT>
__ostream_type&
_M_insert(_ValueT __v);
};
當然，我們還可以保留小數位數呢
切記要加入<iomanip.h>!!!
例子1.3 保留小數位數
#include <iostream>
#include <iomanip>//切記加入這行程式碼
using namespace std;
int main()
{
double a=3.14159;//double函數的意思是定義一個小數變數，變數的概唸的double的概念後續我都會講的
cout<<fixed<<setprecision(3)<<a;//保留三位小數
/*或者這樣：*/
cout<<setprecision(4)<<a;//保留三位小數
return 0;
}
好吧，上文出現了double，我就把基礎變數都和大家微微分享一下，以後會着重講
int
int函數是用來定義整形變數的，舉個例子: int a=3;
float
定義小數變數
-char
定義字元變數
好了，分享到此j結束，我們下期再見，拜拜┏(＾0＾)┛