C++ - Streams

Streams

iostream

• Open a file, then start appending to the file:

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std::ofstream ofs(filename, std::ios::out | std::ios::app);

• How to add a custom print function?

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inline std::ostream &operator<<(std::ostream &os, const Sophus::SE3d &pose) {
    os << "  [SO3] " << pose.so3().log().transpose();
    os << "  [Translation] " << pose.translation().transpose() << "\n";
    return os;
}

std::string and std::stringstream

Strings

• Raw C strings (char arrays) should always terminate with a null character, ‘\0’. Fun fact about char arrays: nullptr is inserted at the back of the array. This is a convention for argv

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char str[] = "a string";
printf("%s", str);  // searches for "\0" to terminate

char *strs[] = {"str1", "str2", nullptr};
for (size_t i = 0; strs[i] != nullptr; ++i){
    // uses nullptr to terminate string array.  
    printf("%s", strs[i]);
}

Substring

• std::string substr(size_t pos = 0, size_t len = std::string::npos) const;
  • std::string::npos is optional. If not provided, it will be the full string size. ```cpp

std::string str = “Hello, World!”; std::string sub1 = str.substr(0, 5); std::string sub3 = str.substr(7);

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### Best way for concactenate variables of different types - use `stringstream`

```cpp
std::ostringstream oss;
oss << "index are not matching! " << idx << "|" << match.idx_in_this_cloud;
throw std::runtime_error(oss.str());

StringStream Operations

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#include <iostream>
#include <sstream>
#include <vector>
using namespace std;
int main()
{
    string str = "(1,2,3)";
    stringstream ss(str);   // read ss 
    // cout<<ss.str()<<endl;
    ss <<"4,5"; // I now see 4,52,3)??
    cout<<ss.str()<<endl;
    ss << str;
    
    stringstream ss2(str.substr(1, str.size()-2));   
    string num;
    char delim=',';
    std::vector<int> vec;
    while(getline(ss2, num, delim)){    // getline 
        vec.push_back(stoi(num));   // in #include <cstdlib>
    }
    return 0;
}

• Initialization: std::stringstream ss(str). Internally, ss has a buffer. This sets the internal buffer
• << is pronounced as the “insertion operator”. Once you call that, the internal buffer starts from 0 position
• bool done_line = getline(ss2, num, delim) is the powerful tool that parses a line by delimeters
• stoi() is the powerful tool to convert a string into an integer

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