1.简介
RapidJSON 是一个 C++ 的 JSON 解析库，由腾讯开源。
- 支持 SAX 和 DOM 风格的 API，并且可以解析、生成和查询 JSON 数据。
- RapidJSON 快。它的性能可与strlen() 相比。可支持 SSE2/SSE4.2 加速。
- RapidJSON 独立。它不依赖于 BOOST 等外部库。它甚至不依赖于STL。
- RapidJSON 对内存友好。在大部分 32/64 位机器上，每个 JSON 值只占 16字节（除字符串外）。它预设使用一个快速的内存分配器，令分析器可以紧凑地分配内存。
- RapidJSON 对 Unicode 友好。它支持UTF-8、UTF-16、UTF-32 (大端序／小端序)，并内部支持这些编码的检测、校验及转码。例如，RapidJSON 可以在分析一个。
- RapidJSON 是跨平台的。
2.环境搭建
下载地址：https://github.com/Tencent/rapidjson/tree/v1.1.0
这里使用的版本1.1.0
下载完成之后解压目录如下，将整个include目录拷贝到我们的工程目录下。
拷贝完成之后如下图所示：
配置文件路径。C/C++ ->常规 ->附加包含目录
3.代码示例
DOM接口示例
cpp
#include "rapidjson/document.h" // rapidjson's DOM-style API
#include "rapidjson/prettywriter.h" // for stringify JSON
#include <iostream>
using namespace rapidjson;
int main()
{
// 1. Parse a JSON text string to a document.
const char json[] = " { \"hello\" : \"world\", \"t\" : true , \"f\" : false, \"n\": null, \"i\":123, \"pi\": 3.1416, \"a\":[1, 2, 3, 4] } ";
printf("Original JSON:\n %s\n", json);
Document document; // Default template parameter uses UTF8 and MemoryPoolAllocator.
// In-situ parsing, decode strings directly in the source string. Source must be string.
char buffer[sizeof(json)];
memcpy(buffer, json, sizeof(json));
if (document.ParseInsitu(buffer).HasParseError())
return 1;
printf("\nParsing to document succeeded.\n");
// 2. Access values in document.
printf("\nAccess values in document:\n");
assert(document.IsObject()); // Document is a JSON value represents the root of DOM. Root can be either an object or array.
assert(document.HasMember("hello"));
assert(document["hello"].IsString());
printf("hello = %s\n", document["hello"].GetString());
// Since version 0.2, you can use single lookup to check the existing of member and its value:
Value::MemberIterator hello = document.FindMember("hello");
assert(hello != document.MemberEnd());
assert(hello->value.IsString());
assert(strcmp("world", hello->value.GetString()) == 0);
(void)hello;
assert(document["t"].IsBool()); // JSON true/false are bool. Can also uses more specific function IsTrue().
printf("t = %s\n", document["t"].GetBool() ? "true" : "false");
assert(document["f"].IsBool());
printf("f = %s\n", document["f"].GetBool() ? "true" : "false");
printf("n = %s\n", document["n"].IsNull() ? "null" : "?");
assert(document["i"].IsNumber()); // Number is a JSON type, but C++ needs more specific type.
assert(document["i"].IsInt()); // In this case, IsUint()/IsInt64()/IsUint64() also return true.
printf("i = %d\n", document["i"].GetInt()); // Alternative (int)document["i"]
assert(document["pi"].IsNumber());
assert(document["pi"].IsDouble());
printf("pi = %g\n", document["pi"].GetDouble());
{
const Value& a = document["a"]; // Using a reference for consecutive access is handy and faster.
assert(a.IsArray());
for (SizeType i = 0; i < a.Size(); i++) // rapidjson uses SizeType instead of size_t.
printf("a[%d] = %d\n", i, a[i].GetInt());
int y = a[0].GetInt();
(void)y;
// Iterating array with iterators
printf("a = ");
for (Value::ConstValueIterator itr = a.Begin(); itr != a.End(); ++itr)
printf("%d ", itr->GetInt());
printf("\n");
}
// Iterating object members
static const char* kTypeNames[] = { "Null", "False", "True", "Object", "Array", "String", "Number" };
for (Value::ConstMemberIterator itr = document.MemberBegin(); itr != document.MemberEnd(); ++itr)
printf("Type of member %s is %s\n", itr->name.GetString(), kTypeNames[itr->value.GetType()]);
// 3. Modify values in document.
// Change i to a bigger number
{
uint64_t f20 = 1; // compute factorial of 20
for (uint64_t j = 1; j <= 20; j++)
f20 *= j;
document["i"] = f20; // Alternate form: document["i"].SetUint64(f20)
assert(!document["i"].IsInt()); // No longer can be cast as int or uint.
}
// Adding values to array.
{
Value& a = document["a"]; // This time we uses non-const reference.
Document::AllocatorType& allocator = document.GetAllocator();
for (int i = 5; i <= 10; i++)
a.PushBack(i, allocator); // May look a bit strange, allocator is needed for potentially realloc. We normally uses the document's.
// Fluent API
a.PushBack("Lua", allocator).PushBack("Mio", allocator);
}
// Making string values.
// This version of SetString() just store the pointer to the string.
// So it is for literal and string that exists within value's life-cycle.
{
document["hello"] = "rapidjson"; // This will invoke strlen()
// Faster version:
// document["hello"].SetString("rapidjson", 9);
}
// This version of SetString() needs an allocator, which means it will allocate a new buffer and copy the the string into the buffer.
Value author;
{
char buffer2[10];
int len = sprintf(buffer2, "%s %s", "Milo", "Yip"); // synthetic example of dynamically created string.
author.SetString(buffer2, static_cast<SizeType>(len), document.GetAllocator());
// Shorter but slower version:
// document["hello"].SetString(buffer, document.GetAllocator());
// Constructor version:
// Value author(buffer, len, document.GetAllocator());
// Value author(buffer, document.GetAllocator());
memset(buffer2, 0, sizeof(buffer2)); // For demonstration purpose.
}
// Variable 'buffer' is unusable now but 'author' has already made a copy.
document.AddMember("author", author, document.GetAllocator());
assert(author.IsNull()); // Move semantic for assignment. After this variable is assigned as a member, the variable becomes null.
// 4. Stringify JSON
printf("\nModified JSON with reformatting:\n");
StringBuffer sb;
PrettyWriter<StringBuffer> writer(sb);
document.Accept(writer); // Accept() traverses the DOM and generates Handler events.
puts(sb.GetString());
return 0;
}运行结果：
SAX接口示例：
cpp
#include "rapidjson/document.h" // rapidjson's DOM-style API
#include "rapidjson/prettywriter.h" // for stringify JSON
#include "rapidjson/reader.h"
#include "rapidjson/error/en.h"
#include <iostream>
#include <string>
#include <map>
using namespace std;
using namespace rapidjson;
typedef map<string, string> MessageMap;
#if defined(__GNUC__)
RAPIDJSON_DIAG_PUSH
RAPIDJSON_DIAG_OFF(effc++)
#endif
#ifdef __clang__
RAPIDJSON_DIAG_PUSH
RAPIDJSON_DIAG_OFF(switch - enum)
#endif
struct MessageHandler : public BaseReaderHandler<UTF8<>, MessageHandler>
{
MessageHandler() : messages_(), state_(kExpectObjectStart), name_() {}
bool StartObject()
{
switch (state_)
{
case kExpectObjectStart:
state_ = kExpectNameOrObjectEnd;
return true;
default:
return false;
}
}
bool String(const char* str, SizeType length, bool)
{
switch (state_)
{
case kExpectNameOrObjectEnd:
name_ = string(str, length);
state_ = kExpectValue;
return true;
case kExpectValue:
messages_.insert(MessageMap::value_type(name_, string(str, length)));
state_ = kExpectNameOrObjectEnd;
return true;
default:
return false;
}
}
bool EndObject(SizeType) { return state_ == kExpectNameOrObjectEnd; }
bool Default() { return false; } // All other events are invalid.
MessageMap messages_;
enum State
{
kExpectObjectStart,
kExpectNameOrObjectEnd,
kExpectValue
}state_;
std::string name_;
};
#if defined(__GNUC__)
RAPIDJSON_DIAG_POP
#endif
#ifdef __clang__
RAPIDJSON_DIAG_POP
#endif
static void ParseMessages(const char* json, MessageMap& messages)
{
Reader reader;
MessageHandler handler;
StringStream ss(json);
if (reader.Parse(ss, handler))
messages.swap(handler.messages_); // Only change it if success.
else
{
ParseErrorCode e = reader.GetParseErrorCode();
size_t o = reader.GetErrorOffset();
cout << "Error: " << GetParseError_En(e) << endl;;
cout << " at offset " << o << " near '" << string(json).substr(o, 10) << "...'" << endl;
}
}
int main()
{
MessageMap messages;
const char* json1 = "{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\" }";
cout << json1 << endl;
ParseMessages(json1, messages);
for (MessageMap::const_iterator itr = messages.begin(); itr != messages.end(); ++itr)
cout << itr->first << ": " << itr->second << endl;
cout << endl << "Parse a JSON with invalid schema." << endl;
const char* json2 = "{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\", \"foo\" : {} }";
cout << json2 << endl;
ParseMessages(json2, messages);
return 0;
}