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### jsoncons::jsonpath::make_expression
```cpp
#include <jsoncons_ext/jsonpath/jsonpath.hpp>
```
```cpp
template <typename Json>
jsonpath_expression<Json> make_expression(const Json::string_view_type& expr); (until 0.164.0)
```
```cpp
template <typename Json> (1)
jsonpath_expression<Json> make_expression(const Json::string_view_type& expr,
const custom_functions<Json>& funcs = custom_functions<Json>()); (since 0.164.0)
```
```cpp
template <typename Json>
jsonpath_expression<Json> make_expression(const Json::string_view_type& expr,
std::error_code& ec); (2)
```
```cpp
template <typename Json>
jsonpath_expression<Json> make_expression(const Json::string_view_type& expr,
const custom_functions<Json>& funcs, std::error_code& ec); (3) (since 0.164.0)
```
```cpp
template <typename Json,typename TempAllocator> (4) (since 0.170.0)
jsonpath_expression<Json> make_expression(const allocator_set<Json::allocator_type,TempAllocator>& alloc_set,
Json::string_view_type& expr,
const custom_functions<Json>& funcs = custom_functions<Json>());
```
```cpp
template <typename Json,typename TempAllocator> (5) (since 0.170.0)
jsonpath_expression<Json> make_expression(const allocator_set<Json::allocator_type,TempAllocator>& alloc_set,
Json::string_view_type& expr, std::error_code& ec);
```
```cpp
template <typename Json,typename TempAllocator> (6) (since 0.170.0)
jsonpath_expression<Json> make_expression(const allocator_set<Json::allocator_type,TempAllocator>& alloc_set,
Json::string_view_type& expr,
const custom_functions<Json>& funcs, std::error_code& ec);
```
(1) Makes a [jsonpath_expression](jsonpath_expression.md) from the JSONPath expression `expr`.
(2-3) Makes a [jsonpath_expression](jsonpath_expression.md) from the JSONPath expression `expr`.
(4-6) Same as (1-3) except that `alloc` is used to allocate memory during expression compilation and evaluation.
#### Parameters
<table>
<tr>
<td><code>expr</code></td>
<td>JSONPath expression string</td>
</tr>
<tr>
<td><code>funcs</code></td>
<td>Custom functions</td>
</tr>
<tr>
<td><code>ec</code></td>
<td>out-parameter for reporting errors in the non-throwing overload</td>
</tr>
</table>
#### Return value
Returns a [jsonpath_expression](jsonpath_expression.md) object that represents the JSONPath expression.
#### Exceptions
(1) throws a [jsonpath_error](jsonpath_error.md) if JSONPath compilation fails.
(2) sets the out-parameter `ec` to the [jsonpath_error_category](jsonpath_errc.md) if JSONPath compilation fails.
### Examples
#### Custom functions
```cpp
#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpath/jsonpath.hpp>
using json = jsoncons::json;
namespace jsonpath = jsoncons::jsonpath;
template <typename Json>
class my_custom_functions : public jsonpath::custom_functions<Json>
{
public:
my_custom_functions()
{
this->register_function("divide", // function name
2, // number of arguments
[](jsoncons::span<const jsonpath::parameter<Json>> params,
std::error_code& ec) -> Json
{
const Json& arg0 = params[0].value();
const Json& arg1 = params[1].value();
if (!(arg0.is_number() && arg1.is_number()))
{
ec = jsonpath::jsonpath_errc::invalid_type;
return Json::null();
}
return Json(arg0.as<double>() / arg1.as<double>());
}
);
}
};
int main()
{
json root = json::parse(R"([{"foo": 60, "bar": 10},{"foo": 60, "bar": 5}])");
auto expr = jsonpath::make_expression<json>("$[?(divide(@.foo, @.bar) == 6)]",
my_custom_functions<json>());
json result = expr.evaluate(root);
std::cout << result << "\n\n";
}
```
Output:
```
[{"bar": 10,"foo": 60}]
```
#### make_expression with stateful allocator
```cpp
#include <string_view> // Assuming C++ 17
#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpath/jsonpath.hpp>
#include "free_list_allocator.hpp" // for free_list_allocator
using my_alloc = free_list_allocator<char>; // an allocator with a single-argument constructor
using my_json = jsoncons::basic_json<char,jsoncons::sorted_policy,my_alloc>;
namespace jsonpath = jsoncons::jsonpath;
int main()
{
auto alloc = my_alloc(1);
// until 0.171.0
// jsoncons::json_decoder<my_json,my_alloc> decoder(jsoncons::result_allocator_arg,
// alloc, alloc);
// since 0.170.1
jsoncons::json_decoder<my_json,my_alloc> decoder(alloc, alloc);
std::ifstream is(/*path_to_books_file*/);
jsoncons::basic_json_reader<char,jsoncons::stream_source<char>,my_alloc> reader(is, decoder, alloc);
reader.read();
my_json doc = decoder.get_result();
std::string_view p{"$.books[?(@.category == 'fiction')].title"};
auto expr = jsonpath::make_expression<my_json>(combine_allocators(alloc), p);
auto result = expr.evaluate(doc);
std::cout << pretty_print(result) << "\n\n";
}
```
Output:
```json
[
"A Wild Sheep Chase",
"The Night Watch",
"The Comedians"
]
```
|
