include: co/fastream.h.


fastream is used to replace std::ostringstream in the C++ standard library. The performance of std::ostringstream is poor which may be several times slower than snprintf, and fastream is about 10~30 times faster than snprintf on different platforms.


1. constexpr fastream() noexcept;
2. explicit fastream(size_t cap);
3. fastream(fastream&& s) noexcept;
  • 1, the default constructor, which creates an empty fastream object without any memory allocation.

  • 2, uses the parameter cap to specify the initial capacity of fastream.

  • 3, the move constructor.

  • Example

fastream s;              // empty fastream, no memory allocated
fastream s(1024);        // Pre-allocate 1k bytes
fastream x(std::move(s); // move construction, s becomes empty


fastream& operator=(fastream&& s) noexcept;
  • Move assignment, the content of s is transferred to fastream, and s itself becomes empty.

  • Example

fastream s(32);
fastream x;
x = std::move(s); // x capacity -> 32, s -> empty



char& back();
const char& back() const;
  • This method returns a reference to the last character in fastream.
If fastream is empty, calling this method will cause undefined behavior.
  • Example
fastream s;
char c = s.back(); // c ='o'
s.back() ='x';     // s -> "hellx"


char& front();
const char& front() const;
  • This method returns the reference of the first character in fastream.
If fastream is empty, calling this method will cause undefined behavior.
  • Example
fastream s;
char c = s.front(); // c ='h'
s.front() ='x';     // s -> "xello"


char& operator[](size_t n);
const char& operator[](size_t n) const;
  • This method returns the reference of the nth character in fastream.
If n is out of a reasonable range, calling this method will cause undefined behavior.
  • Example
fastream s;
char c = s[1]; // c ='e'
s[1] ='x';     // s -> "hxllo"



size_t capacity() const noexcept;
  • This method returns the capacity of fastream.


const char* c_str() const;
  • This method gets the equivalent C-style string (null terminated).
Writing to the character array accessed through c_str() is undefined behavior.


char* data() noexcept;
const char* data() const noexcept;
  • This method is similar to c_str(), but it does not guarantee that the string ends with ‘\0’.


bool empty() const noexcept;
  • This method determines whether fastream is empty.


size_t size() const noexcept;
  • This method returns the length of data in fastream.


fastring str() const;
  • This method returns a copy of fastream’s internal data in the form of fastring.

  • Example

fastream s;
fastring x = s.str(); // x = "hello"



1. void clear();
2. void clear(char c);
  • This method sets the size of fastream to 0, and the capacity remains unchanged.
  • The second is similar to the first, except that it will fill the memory with character c before setting its size to 0.
The second one is added in v3.0.1, which can be used to clear sensitive information in memory.


void ensure(size_t n);
  • This method ensures that the remaining memory of fastream can hold at least n characters.


void reserve(size_t n);
  • This method ensures that the capacity of fastream is at least n.
  • When n is less than the original capacity, the capacity remains unchanged.


void reset();
  • Added in v2.0.3. Clear fastream and free the memory.


1. void resize(size_t n);
2. void resize(size_t n, char c);
  • This method sets the size of fastream to n.

  • When n is greater than the original size, it will expand size to n. In the 1st version, the content of the extended part is undefined. The 2nd version will fill the extended part with character c.

  • Example

fastream s;
s.resize(3);    // s -> "hel"
char c = s[5];  // c is an uncertain random value

s.resize(6, 0);
c = s[5];       // c is '\0'


void swap(fastream& s) noexcept;
void swap(fastream&& s) noexcept;
  • Swap the contents of two fastreams, only the internal pointer, capacity, and size are exchanged.

  • Example

fastream s(32);
fastring x(64);
s.swap(x); // s: cap -> 64, x: cap -> 32



1.  fastream& append(const void* s, size_t n);
2.  fastream& append(const char* s);
3.  fastream& append(const fastring& s);
4.  fastream& append(const std::string& s);
5.  fastream& append(const fastream& s);

6.  fastream& append(size_t n, char c);
7.  fastream& append(char c);
8.  fastream& append(signed char v)
9. fastream& append(unsigned char c);

10. fastream& append(uint16 v);
11. fastream& append(uint32 v);
12. fastream& append(uint64 v);
  • 1, appends a byte sequence of length n.
  • 2-4, append string s.
  • 5, appends a fastream, s can be the fastream itself that performs the append operation.
  • 6, append n characters c.
  • 7-9, append a single character c.
  • 10-12, equivalent to append(&v, sizeof(v)).
Since v3.0.1, it is ok that the parameter s in 1 and 2 overlaps with the internal memory of fastream.
v3.0.2 removed fastream& append(char c, size_t n);.
  • Example
fastream s;
int32 i = 7;
char buf[8];

s.append("xx");      // Append C string
s.append(s);         // append itself, s -> "xxxx"
s.append(buf, 8);    // Append 8 bytes
s.append('c');       // Append a single character
s.append(100,'c');   // Append 100'c'

s.append(&i, 4);     // Append 4 bytes
s.append(i);         // Append 4 bytes, same as above
s.append((int16)23); // Append 2 bytes

s.append(s.c_str() + 1); // ok since v3.0.1


fastream& append_nomchk(const void* s, size_t n);
fastream& append_nomchk(const char* s)
  • Similar to append(), but will not check if s overlaps with the internal memory.
This method cannot be used if s may overlap with the internal memory of fastream.


template<typename X, typename ...V>
fastream& cat(X&& x, V&& ... v);
  • Added in v2.0.3. Concatenate any number of elements to fastream.

  • This method appends elements in the parameters to fastream one by one through operator<<.

  • Example

fastream s;
s << "hello";'', 23, "xx", false); // s -> "hello 23xxfalse"


fastream& operator<<(bool v);

fastream& operator<<(char v);
fastream& operator<<(signed char v);
fastream& operator<<(unsigned char v);

fastream& operator<<(short v);
fastream& operator<<(unsigned short v);
fastream& operator<<(int v);
fastream& operator<<(unsigned int v);
fastream& operator<<(long v);
fastream& operator<<(unsigned long v);
fastream& operator<<(long long v);
fastream& operator<<(unsigned long long v);

fastream& operator<<(double v);
fastream& operator<<(float v);
fastream& operator<<(const dp::_fpt& v);

fastream& operator<<(const void* v);
fastream& operator<<(std::nullptr_t);

fastream& operator<<(const char* s);
fastream& operator<<(const signed char* s);
fastream& operator<<(const unsigned char* s);
fastream& operator<<(const fastring& s);
fastream& operator<<(const std::string& s);
fastream& operator<<(const fastream& s);
  • Format value of bool, char, integer type, floating point type, pointer type or string type, and append it to fastream.

  • operator<<(const dp::_fpt& v) is used to output formatted floating-point number, which the effective decimal places can be specified.

  • Example

fastream s;
s << 'x';             // s -> "x"
s << s;               // s -> "xx"           (append itself)
s << false;           // s -> "xxfalse"

s << "hello " << 23;  // s -> "hello 23"
s << (s.c_str() + 6); // s -> "hello 2323"   (append part of s)

s << 3.1415;          // s -> "3.1415"

s << (void*)32;       // s -> "0x20"

Specify the number of valid decimal places
coost provides dp::_1, dp::_2, ..., dp::_16, dp::_n to set the number of effective decimal places of floating-point numbers.

fastream s;
s << dp::_2(3.1415);   // "3.14
s << dp::_3(3.1415);   // "3.141"
s << dp::_n(3.14, 1);  // "3.1", equivalent to dp::_1(3.14)

co.log is implemented based on fastream, so we can also use the above methods when printing logs.

LOG << dp::_2(3.1415);