逗号的左侧操作数没有作用？

Question

我遇到一些麻烦，此警告消息

，它是一个模板容器类

int k = 0, l = 0; 
    for (k =(index+1), l=0; k < sizeC, l < (sizeC-index); k++,l++){ 
     elements[k] = arryCpy[l]; 
    } 
    delete[] arryCpy; 

内实现，这是警告我得到

cont.h: In member function `void Container<T>::insert(T, int)': 
cont.h:99: warning: left-hand operand of comma has no effect 
cont.h: In member function `void Container<T>::insert(T, int) [with T = double]': 
a5testing.cpp:21: instantiated from here 
cont.h:99: warning: left-hand operand of comma has no effect 
cont.h: In member function `void Container<T>::insert(T, int) [with T = std::string]': 
a5testing.cpp:28: instantiated from here 
cont.h:99: warning: left-hand operand of comma has no effect 
>Exit code: 0 

Answer 1

逗号表达式a,b,c,d,e类似于

{ 
    a; 
    b; 
    c; 
    d; 
    return e; 
} 

因此，k<sizeC, l<(sizeC - index)将只返回l < (sizeC - index)。

要组合条件，请使用&&或||。

k < sizeC && l < (sizeC-index) // both must satisfy 
k < sizeC || l < (sizeC-index) // either one is fine. 

Answer 2

更改为：

for (k =(index+1), l=0; k < sizeC && l < (sizeC-index); k++,l++){ 

当你有一个逗号表达式被评估时，最右边的参数被返回，所以你的：

k < sizeC, l < (sizeC-index) 

表达式评估为：

l < (sizeC-index) 

并因此错过

k < sizeC 

使用&&的条件，而不是结合起来。

Answer 3

表达式k < sizeC, l < (sizeC-index)只返回右侧测试的结果。使用&&结合试验：

k < sizeC && l < (sizeC-index)