C＆＃x2B;＆＃x2B;将字符串作为值或const String＆amp传递。
说我的功能很简单：
struct Data {
string name;
string value; // Can be very long
};
// Use Data directly to avoid copy. (Not Data*)
unordered_map<size_t, Data> g_data;
// Should I use "const string& name, const string& value"?
void addData(string name, string value) {
// Should I use any std::move? Like:
// ... = {std::move(name), std::move(value)};
g_data[hash(name, value)] = {name, value};
}
// Returning as string seems obvious.
string generateData() {
// Some computation...
return value;
}
void test() {
addData(generateName(), generateDatA());
}
我知道上述代码有效。但是我想知道我是否应该在adddata中使用const String＆amp;？我也想知道std ::移动可以使其更有效吗？
我至少使用C ++ 14，并且还启用了C ++ 17。
Say I have such simple functions:
struct Data {
string name;
string value; // Can be very long
};
// Use Data directly to avoid copy. (Not Data*)
unordered_map<size_t, Data> g_data;
// Should I use "const string& name, const string& value"?
void addData(string name, string value) {
// Should I use any std::move? Like:
// ... = {std::move(name), std::move(value)};
g_data[hash(name, value)] = {name, value};
}
// Returning as string seems obvious.
string generateData() {
// Some computation...
return value;
}
void test() {
addData(generateName(), generateDatA());
}
I know that the above code works. But I'm wondering whether I should use const string& in addData? I also wonder std::move can make it more efficient?
I'm using at least C++14, and C++17 is also enabled.
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评论（2）
是的，您应该使用
std ::移动，但不像这样。提出的代码将尝试从字符串中移动（pre-c ++ 17，如果已经从那时已经移动，请参见规则＃20 在这里）。您应该将哈希进行预测并将其存储在一个变量中：
您不应通过const Reference传递
字符串名称，字符串值，因为它会阻止您移动它们。Yes, you should use
std::move, but not like this. The proposed piece of code would try to hash moved-from strings (pre-C++17 it was unspecified if the strings would be already moved from at that point, see rule #20 here).You should pre-calculate the hash and store it in a variable:
You should NOT pass
string name, string valueby const reference, since it would prevent you from moving them.对于第一个问题：是的，使用参考更好，然后为每个
adddata函数调用制作复制。如果您不使用它，则每个调用将创建名称和值的（不必要的）副本。对于第二个问题：是的，移动可以更有效。
编辑：
有关我的答案的补充，请参阅下面的 @user17732522评论。
For the first question: yes, use a reference is better then make a copy for every
addDatafunction call. If you don't use it, each call will create a (unnecessary) copy of name and value.For the second question: yes, move can be more efficient.
EDIT:
For a complement of my answer, please refer to @user17732522 comment below.