BigInt的实现——C++编程风格读书笔记

C++编程风格这本书前面一些章节都觉得很简明易懂，但是读到效率这一章是才充分认识到读别人的代码还是很痛苦的一件事。书中给出的需要改进的初始类如下：

class BigInt  
{  
private:  
    char* digits;  
    unsigned ndigits;  
    BigInt(char *d,unsigned n)  
    {  
        digits = d;  
        ndigits = n;  
    }  
    friend class DigitStream;  
public:  
    BigInt(const char*);  
    BigInt(unsigned n = 0);  
    BigInt(const BigInt&);  
    void operator=(const BigInt&);  
    BigInt operator+(const BigInt&) const;  
    void print(FILE* f = stdout) const;  
    ~BigInt() {delete [] digits;}  
};  
  
class DigitStream  
{  
private:  
    char* dp;  
    unsigned nd;  
public:  
    DigitStream(const BigInt& n)  
    {  
        dp = n.digits;  
        nd = n.ndigits;  
    }  
    unsigned operator++()  
    {  
        if(nd == 0)  
            return 0;  
        else  
        {  
            nd--;  
            return *dp++;  
        }  
    }  
};  
  
void BigInt::print(FILE* f) const  
{  
    for(int i = ndigits - 1;i >= 0;i --)  
        fprintf(f,"%c",digits[i]+'0');  
}  
void BigInt::operator=(const BigInt& n)  
{  
    if (this == &n) return;  
    delete [] digits;  
    unsigned i = n.ndigits;  
    digits = new char[ndigits = i];  
    char* p = digits;  
    char* q = n.digits;  
    while(i--) *p++ = *q++;  
}  
BigInt BigInt::operator+(const BigInt& n) const  
{  
    unsigned maxDigits = (ndigits > n.ndigits ? ndigits : n.ndigits) + 1;  
    char* sumPtr = new char[maxDigits];  
    BigInt sum(sumPtr,maxDigits);  
    DigitStream a(*this);  
    DigitStream b(n);  
    unsigned i = maxDigits;  
    unsigned carry = 0;  
    while (i --)   
    {  
        *sumPtr = (++a) + (++b) + carry;  
        if(*sumPtr >= 10)  
        {  
            carry = 1;  
            *sumPtr -= 10;  
        }  
        else carry = 0;  
        sumPtr++;  
    }  
    return sum;  
}  
  
BigInt::BigInt(unsigned n)  
{  
    char d[3*sizeof(unsigned)+1];  
    char *dp = d;  
    ndigits = 0;  
    do  
    {  
        *dp++ = n % 10;  
        n /= 10;  
        ndigits++;    
    } while(n > 0);  
    digits = new char[ndigits];  
    for(register int i = 0;i < ndigits;i++)  
        digits[i] = d[i];  
}  
  
BigInt::BigInt(const BigInt& n)  
{  
    unsigned i = n.ndigits;  
    digits = new char[ndigits = i];  
    char* p = digits;  
    char* q = n.digits;  
    while(i--) *p++ = *q++;  
}  
  
BigInt::BigInt(const char* digitString)  
{  
    unsigned n = strlen(digitString);  
    if(n != 0)  
    {  
        digits = new char[ndigits=n];  
        char* p = digits;  
        const char* q = &digitString[n];  
        while(n--) *p++ = *--q - '0';  
    }  
    else  
    {  
        digits = new char[ndigits=1];  
        digits[0] = 0;  
    }  
}  

View Code