premature optimization

最新推荐文章于 2023-06-07 11:03:01 发布

我不是大明

最新推荐文章于 2023-06-07 11:03:01 发布

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分类专栏： C++ 文章标签： optimization function scroll types float c++

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博客展示了C++中`test`函数的代码，探讨了变量赋值与初始化的情况。对于简单类型如`double`、`float`，赋值和初始化差异不大，但对于有耗时构造函数的类，二者可能存在明显不同。

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在性能上下面的二个函数有什么不同呢？哪个运行效率更高呢？

double test()
{
double a,b,c;
for(int i=0; i<n; i++){
a = i*10;
b = a-i;
c += a*b;
}
return c;

}

or:

double test()
{
double c;

for(int i=0; i<n; i++){
double a = i*10;
double b = a-i;
c += a*b;
}
return c;

}

Answer is : the later

consider if n == 0. if n==0 for your second example the varibles would
never have to be allocated.

If they will always be allocated (n > 0) then the difference is assignment
.vs. initialization. For simple types (double, float, etc...) it probably
doesn't make much difference. For classes, however, it can if they have
constructors that take time.

The general rule in C++ is delcare varaibles just before you use them. One
of the main reasons is so you know what the variable is declared. If a
function is large and you come across
a = 10;
what is a? Is it int? double? char? a class? You'd have to scroll up to
the top of the function to find out. However
double a = 10;
makes it easier to read/maintain the code IMO.