string and c_str C++

1 string (C++)

1.1 initialization

#include <iostream>
#include <string>

using namespace std;

int main()
{

//   string s1(); function declaration
  string s1;            //default constructor
  string s2(s1);        //copy constructor
  s2 = "abcdf";
  string s3(s2, 2);     //substring constructor
  cout<<s2<<" "<<s3<<endl;
  string s4("test");    //from c-string
  string s5("test", 2); //from sequence
  string s6(10, 'a');   //fill constructor
  string s7(s6.begin(), s6.end());
  cout<<s1<<"\n"<<s2<<"\n"<<s3<<"\n";
  cout<<s4<<"\n"<<s5<<"\n"<<s6<<"\n";
  cout<<s7<<"\n";
  
  return 0;

}

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1.2 size vs length vs capacity vs max_size

dynamic size during the runtime

size和length是相同的，可以互换使用 - how many characters actually exist in the string

capacity - the maximum number of characters that the string can currently hold without having to grow

max_size - the maximum length the string can reach

#include <iostream>
#include <string>

using namespace std;

int main()
{

  string str ("Test~");
  cout << "size: " << str.size() << "\n";
  cout << "length: " << str.length() << "\n";
  cout << "capacity: " << str.capacity() << "\n";
  cout << "max_size: " << str.max_size() << "\n";
  return 0;

}

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string is not null-terminated so we can add '\0' into the string.

#include <iostream>
#include <string>

using namespace std;

int main()
{

  string s1 = "ab\0\0cd";   // s1 contains "ab", using string literal
  string s2{"ab\0\0cd", 4}; // s2 contains "ab\0\0cd", using different ctr
  string s3 = "ab\0\0cd"s;  // s3 contains "ab\0\0cd", using ""s operator (since C++14)
  
  cout << s1 << endl;
  cout << "size: " << s1.size() << "\n";
  cout << "length: " << s1.length() << "\n";
  cout << "capacity: " << s1.capacity() << "\n";
  cout << "max_size: " << s1.max_size() << "\n\n";
  
  cout << s2 << endl;
  cout << "size: " << s2.size() << "\n";
  cout << "length: " << s2.length() << "\n";
  cout << "capacity: " << s2.capacity() << "\n";
  cout << "max_size: " << s2.max_size() << "\n\n";
  
  cout << s3 << endl;
  cout << "size: " << s3.size() << "\n";
  cout << "length: " << s3.length() << "\n";
  cout << "capacity: " << s3.capacity() << "\n";
  cout << "max_size: " << s3.max_size();
  
  return 0;

}

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#include <iostream>
#include <string>

using namespace std;

int main()
{

    string s1 = "this is a string~";
    s1[2] = '\0';
    string s2 = "\0also a string~";
    cout<<"the length of s1 - ("<<s1<<") : "<<s1.size()<<endl;
    cout<<"the length of s2 - ("<<s2<<") : "<<s2.size()<<endl;
  
    return 0;

}

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1.3 string -> c_str

#include <iostream>
#include <string>
#include <string.h>

using namespace std;

int main()
{

    string s1 = "string";
    const char* s2 = s1.c_str();  // make sure null-termination
    const char* s3 = s1.data();
    char s4[20];
    strcpy(s4, s1.c_str());

    cout<<s1<<" "<<s1.size()<<" "<<sizeof(s1)<<endl;
    cout<<s2<<" "<<strlen(s2)<<" "<<sizeof(s2)<<endl; // the size of a pointer
    cout<<s3<<" "<<strlen(s3)<<" "<<sizeof(s3)<<endl; // the size of a pointer
    cout<<s4<<" "<<strlen(s4)<<" "<<sizeof(s4)<<endl;
  
    return 0;

}

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2 char[] (C string)

2.1 initialization

#include <iostream>
#include <string>
#include <string.h>

using namespace std;

int main()
{

    char foo1[20];                               // 必须要知道char array的size
    char foo2[] = {};                            // size=0
    char foo3[] = {'a', 'b', 'c', '\0'};         // size=4
    char foo4[] = "";                            // size=1
    char foo5[] = "abc";                         // size=4
    char foo6[20] = "a";
    
    cout<<"foo1: ("<<foo1<<") "<<strlen(foo1)<<" "<<sizeof(foo1)<<endl; // strlen(search for null only) returns a undefined reault here
    cout<<"foo2: ("<<foo2<<") "<<strlen(foo2)<<" "<<sizeof(foo2)<<endl;
    cout<<"foo3: ("<<foo3<<") "<<strlen(foo3)<<" "<<sizeof(foo3)<<endl;
    cout<<"foo4: ("<<foo4<<") "<<strlen(foo4)<<" "<<sizeof(foo4)<<endl;
    cout<<"foo5: ("<<foo5<<") "<<strlen(foo5)<<" "<<sizeof(foo5)<<endl;
    cout<<"foo6: ("<<foo6<<") "<<strlen(foo6)<<" "<<sizeof(foo6)<<endl;

    return 0;

}

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2.2 strlen vs sizeof

strlen - what's size of the string contain, not including null run-time

sizeof - what's the size of the container, including null compile time

2.3 char[] -> string

see below

3 char* (C string)

initialization

#include <iostream>
#include <string>
#include <string.h>

using namespace std;

int main()
{

    char foo[] = "abc";
    char* fo = "abc";
    string foo_str(foo);
    string fo_str(fo);
    
    cout<<foo_str<<" "<<foo_str.size()<<endl;
    cout<<fo_str<<" "<<fo_str.size()<<endl;

    return 0;

}

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