[英]Extracting and joining strings with boost::algorithms::join
I have a collection of a certain struct, and I want to join a string member of each of these structs into a semicolon-separated list. 我有一个特定结构的集合,我想将每个这些结构的字符串成员加入以分号分隔的列表中。 Making good use of the standard algorithms library and Boost, the problem breaks down easily:
充分利用标准算法库和Boost,可以轻松解决问题:
std::transform
to get a collection of the strings, then std::transform
获取字符串的集合,然后 boost::algorithm::join
to join them together. boost::algorithm::join
将它们连接在一起。 However, making a copy of the strings just to feed them to join
is silly, so I attempted to use a reference_wrapper
around the strings as the output for std::transform
. 但是,仅复制字符串以供它们
join
是很愚蠢的,因此我尝试在字符串周围使用reference_wrapper
作为std::transform
的输出。 The corresponding code can be seen here: 相应的代码可以在这里看到:
struct IPAddress {
vector<uint8_t> binaryValue;
string stringValue;
}
// ---snip---
vector<IPAddress> addresses;
// Populate and manipulate the addresses vector
// ---snip---
// Join the list.
vector<reference_wrapper<string>> addressStrings(addresses.size());
transform(begin(addresses), end(addresses), begin(addressStrings),
[](const IPAddress& addr) {
return ref(addr.stringValue); // Expodes
});
return boost::algorithm::join(addressStrings, ";"); // Also explodes
The lambda provided to std::transform
fails to compile, claiming that there is no match for the assignment operator between two reference_wrapper<string>
s, despite that being documented and the entire point of a reference_wrapper
. 提供给
std::transform
的lambda无法编译,声称尽管两个reference_wrapper<string>
都已记录在案 ,但对reference_wrapper
的整个要点也没有匹配。
Then, if I comment out the transform
call, boost::algorithm::join
fails anyways by trying to call a default constructor of reference_wrapper
(which has none) instead of string
. 然后,如果我注释掉了
transform
调用,则boost::algorithm::join
仍然会通过尝试调用reference_wrapper
的默认构造函数(没有)而不是string
来失败。
Am I missing something straightforward, or should I just give up and use a for
loop with iterators? 我是否缺少一些简单明了的东西,还是应该放弃并在迭代器中使用
for
循环?
With a little help from boost ranges you can have your cake and eat it. 在增强范围的一点帮助下,您可以吃蛋糕并食用。
It removes the sting from this mix: the complication is that you want to use an intermediate collection of std::string const&
but, obviously that doesn't work with std::vector
. 它消除了这种混合的麻烦:复杂之处在于,您想使用
std::string const&
的中间集合,但是显然不适用于std::vector
。
So, we drop the intermediate collection, instead using an adapted view ( boost::adaptors::transformed
) and, no need for a lambda either[¹], just use std::mem_fun
: See it Live On Coliru 因此,我们删除中间集合,而不是使用适应的视图 (
boost::adaptors::transformed
),并且也不需要lambda [¹],只需使用std::mem_fun
:观看Live on Coliru
#include <boost/algorithm/string.hpp>
#include <boost/range/adaptors.hpp>
#include <vector>
#include <string>
#include <functional>
struct IPAddress {
std::vector<uint8_t> binaryValue;
std::string stringValue;
};
std::string foo(std::vector<IPAddress> const& addresses)
{
using namespace boost::adaptors;
return boost::algorithm::join(
addresses | transformed(std::mem_fn(&IPAddress::stringValue)),
";");
}
#include <iostream>
int main()
{
std::vector<IPAddress> const addresses {
{ {}, "test1" },
{ {}, "test2" },
};
std::cout << foo(addresses);
}
Prints 打印
test1;test2
[¹] unless stringValue
was an overloaded member function [¹]除非
stringValue
是重载的成员函数
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