fstream与 C 风格(例如fread 和 fwrite )两种读写文件方法的效率比较
2017-04-04 23:07
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为了探录c++ 风格的fstream与 C 风格(例如fread 和 fwrite )两种读写文件的方法的效率,我特意做了两个实验。
我的机器是Windows XP, Visual Studio 2008
1. 测试写文件速度
程序设计思路: 将TEST_SIZE个字符用两种方式写入文件,记录两种方式的耗时。
实验代码:
[cpp] view plain copy
void test_write()
{
const int TEST_SIZE = 10000000 ;
const char* c_plus_write_file = "H://c_plus_write_file.txt";
const char* c_write_file = "H://c_write_file.txt";
cout<<"Test size :" << TEST_SIZE <<endl;
//c++ style writing file
ofstream of(c_plus_write_file);
assert(of);
time_t start, end;
start = clock();
for(int i=0; i < TEST_SIZE; ++i)
{
char tmp[1];
tmp[0] = char(i);
of << tmp[0];
}
end = clock();
of.close();
cout<<"C++ style: "<<end - start <<" ms"<<endl;
//c style writing file
FILE* fp = fopen(c_write_file, "w");
start = clock();
for(int i=0; i < TEST_SIZE; ++i)
{
char tmp[1];
tmp[0] = char(i);
fwrite( tmp, 1, 1, fp);
}
end = clock();
fclose(fp);
cout<<"C style: "<<end - start <<" ms"<<endl;
cin.get();
}
实验结果:
图1
图2
图3
**从图1、2、3,可以看出, ofstream 的 << 运算符 所耗时 是fwrite()的近三倍
把of<<的代码改成了: of.write(tmp,1); 后结果:
实验代码:
[cpp] view plain copy
void test_write()
{
const int TEST_SIZE = 1000000 ;
const char* c_plus_write_file = "H://c_plus_write_file.txt";
const char* c_write_file = "H://c_write_file.txt";
cout<<"Test size :" << TEST_SIZE <<endl;
//c++ style writing file
ofstream of(c_plus_write_file);
assert(of);
time_t start, end;
start = clock();
for(int i=0; i < TEST_SIZE; ++i)
{
char tmp[1];
tmp[0] = char(i);
of.write(tmp,1);
}
end = clock();
of.close();
cout<<"C++ style: "<<end - start <<" ms"<<endl;
//c style writing file
FILE* fp = fopen(c_write_file, "w");
start = clock();
for(int i=0; i < TEST_SIZE; ++i)
{
char tmp[1];
tmp[0] = char(i);
fwrite( tmp, 1, 1, fp);
}
end = clock();
fclose(fp);
cout<<"C style: "<<end - start <<" ms"<<endl;
cin.get();
}
实验结果:
图4
图5
图6
对比图4 和 图1、 图5 和 图2、图6 和 图3, 可以看到 << 运算符没有 ofstream.write(), 快, 但两者还是没有 fwrite() 快
结论: 效率 fwrite() > ofstream.operator<<() > ofstream.write()
3. 下面做读文件的比较:
程序设计思路: 用两种方法去读一个近100M的文本,记录时间。
实验代码:
[cpp] view plain copy
void test_read()
{
const char* read_file = "H://read4.txt";
const int BUF_SIZE = 1024 ;
char buf[BUF_SIZE];
//c++ style writing file
ifstream ifs(read_file,<a href="http://lib.csdn.net/base/ios" class='replace_word' title="iOS知识库" target='_blank' style='color:#df3434; font-weight:bold;'>iOS</a>::binary);
assert(ifs);
time_t start, end;
start = clock();
while(!ifs.eof())
{
ifs.read(buf,BUF_SIZE);
}
end = clock();
ifs.close();
cout<<"C++ style: "<<end - start <<" ms"<<endl;
//c style writing file
FILE* fp = fopen(read_file, "rb");
start = clock();
int len = 0;
do
{
len = fread(buf,1,BUF_SIZE,fp);
//cout<<len<<endl;
}while(len != 0);
end = clock();
fclose(fp);
cout<<"C style: "<<end - start <<" ms"<<endl;
cin.get();
}
实验结果:
图7
结论: 读取一个 100M 的文件, fread() 的效率 是 ifstream.read()的将近十倍! (此结论惊人!)
我的机器是Windows XP, Visual Studio 2008
1. 测试写文件速度
程序设计思路: 将TEST_SIZE个字符用两种方式写入文件,记录两种方式的耗时。
实验代码:
[cpp] view plain copy
void test_write()
{
const int TEST_SIZE = 10000000 ;
const char* c_plus_write_file = "H://c_plus_write_file.txt";
const char* c_write_file = "H://c_write_file.txt";
cout<<"Test size :" << TEST_SIZE <<endl;
//c++ style writing file
ofstream of(c_plus_write_file);
assert(of);
time_t start, end;
start = clock();
for(int i=0; i < TEST_SIZE; ++i)
{
char tmp[1];
tmp[0] = char(i);
of << tmp[0];
}
end = clock();
of.close();
cout<<"C++ style: "<<end - start <<" ms"<<endl;
//c style writing file
FILE* fp = fopen(c_write_file, "w");
start = clock();
for(int i=0; i < TEST_SIZE; ++i)
{
char tmp[1];
tmp[0] = char(i);
fwrite( tmp, 1, 1, fp);
}
end = clock();
fclose(fp);
cout<<"C style: "<<end - start <<" ms"<<endl;
cin.get();
}
实验结果:
图1
图2
图3
**从图1、2、3,可以看出, ofstream 的 << 运算符 所耗时 是fwrite()的近三倍
把of<<的代码改成了: of.write(tmp,1); 后结果:
实验代码:
[cpp] view plain copy
void test_write()
{
const int TEST_SIZE = 1000000 ;
const char* c_plus_write_file = "H://c_plus_write_file.txt";
const char* c_write_file = "H://c_write_file.txt";
cout<<"Test size :" << TEST_SIZE <<endl;
//c++ style writing file
ofstream of(c_plus_write_file);
assert(of);
time_t start, end;
start = clock();
for(int i=0; i < TEST_SIZE; ++i)
{
char tmp[1];
tmp[0] = char(i);
of.write(tmp,1);
}
end = clock();
of.close();
cout<<"C++ style: "<<end - start <<" ms"<<endl;
//c style writing file
FILE* fp = fopen(c_write_file, "w");
start = clock();
for(int i=0; i < TEST_SIZE; ++i)
{
char tmp[1];
tmp[0] = char(i);
fwrite( tmp, 1, 1, fp);
}
end = clock();
fclose(fp);
cout<<"C style: "<<end - start <<" ms"<<endl;
cin.get();
}
实验结果:
图4
图5
图6
对比图4 和 图1、 图5 和 图2、图6 和 图3, 可以看到 << 运算符没有 ofstream.write(), 快, 但两者还是没有 fwrite() 快
结论: 效率 fwrite() > ofstream.operator<<() > ofstream.write()
3. 下面做读文件的比较:
程序设计思路: 用两种方法去读一个近100M的文本,记录时间。
实验代码:
[cpp] view plain copy
void test_read()
{
const char* read_file = "H://read4.txt";
const int BUF_SIZE = 1024 ;
char buf[BUF_SIZE];
//c++ style writing file
ifstream ifs(read_file,<a href="http://lib.csdn.net/base/ios" class='replace_word' title="iOS知识库" target='_blank' style='color:#df3434; font-weight:bold;'>iOS</a>::binary);
assert(ifs);
time_t start, end;
start = clock();
while(!ifs.eof())
{
ifs.read(buf,BUF_SIZE);
}
end = clock();
ifs.close();
cout<<"C++ style: "<<end - start <<" ms"<<endl;
//c style writing file
FILE* fp = fopen(read_file, "rb");
start = clock();
int len = 0;
do
{
len = fread(buf,1,BUF_SIZE,fp);
//cout<<len<<endl;
}while(len != 0);
end = clock();
fclose(fp);
cout<<"C style: "<<end - start <<" ms"<<endl;
cin.get();
}
实验结果:
图7
结论: 读取一个 100M 的文件, fread() 的效率 是 ifstream.read()的将近十倍! (此结论惊人!)
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