C++string字符串拼接遇到的问题及解决

//目的： 把字符串student_ 和A拼接起来
string nameseed = "ABCDE";
string name;
name="student_" +nameseed[0];
cout<<"拼接后的字符串："<<name<<endl;

string nameseed = "ABCDE";
string name;
name = std::string("student_")+ nameseed[0];
cout<<"拼接后的字符串："<<name<<endl;

string nameseed = "ABCDE";
string name;
name = "student_";
name += nameseed[0];
cout << "拼接后的字符串：" << name << endl;

#include <iostream>
#include <string>
#include <sys/time.h>
#include <sstream>
#include <stdio.h>
using namespace std;
#define OUT_IN_REPEATE_NUM 10000
#define IN_REPEATE_NUM 60
string s1="abcedfg";
string s2="hijklmn";
string s3="opqrst";
void  plusTest(string& ret)
{
    for(int i=0; i<IN_REPEATE_NUM; i++)
    {
        ret += s1;
        ret += s2;
        ret += s3;
    }
}
void  appendTest(string& ret)
{
    for(int i=0; i<IN_REPEATE_NUM; i++)
    {
        ret.append(s1);
        ret.append(s2);
        ret.append(s3);
    }
}
void sprintfTest(string& ret)
{
    const size_t length=26*IN_REPEATE_NUM;
    char tmp[length];
    char* cp = tmp;
    size_t strLength=s1.length()+s2.length()+s3.length();
    for(int i=0; i<IN_REPEATE_NUM; i++)
    {
        sprintf(cp,"%s%s%s", s1.c_str(), s2.c_str(),s3.c_str());
        cp+=strLength;
    }
    ret = tmp;
}
void  ssTest(string& ret)
{
    stringstream ss;
    for(int i=0; i<IN_REPEATE_NUM; i++)
    {
        ss<<s1;
        ss<<s2;
        ss<<s3;
    }
    ret = ss.str();
}
int main() {
    string ss, plus, append, sprintf;
    struct timeval sTime, eTime;
    gettimeofday(&sTime, NULL);
    for(int i=0; i<OUT_IN_REPEATE_NUM; i++)
    {
        sprintf="";
        sprintfTest(sprintf);
    }
    gettimeofday(&eTime, NULL);
    long SprintfTime = (eTime.tv_sec-sTime.tv_sec)*1000000+(eTime.tv_usec-sTime.tv_usec); //exeTime 单位是微秒
    gettimeofday(&sTime, NULL);
    for(int i=0; i<OUT_IN_REPEATE_NUM; i++)
    {
        append="";
        appendTest(append);
    }
    gettimeofday(&eTime, NULL);
    long AppendTime = (eTime.tv_sec-sTime.tv_sec)*1000000+(eTime.tv_usec-sTime.tv_usec); //exeTime 单位是微秒
    gettimeofday(&sTime, NULL);
    for(int i=0; i<OUT_IN_REPEATE_NUM; i++)
    {
        ss="";
        ssTest(ss);
    }
    gettimeofday(&eTime, NULL);
    long SsTime = (eTime.tv_sec-sTime.tv_sec)*1000000+(eTime.tv_usec-sTime.tv_usec); //exeTime 单位是微秒
    gettimeofday(&sTime, NULL);
    for(int i=0; i<OUT_IN_REPEATE_NUM; i++)
    {
        plus="";
        plusTest(plus);
    }
    gettimeofday(&eTime, NULL);
    long PlusTime = (eTime.tv_sec-sTime.tv_sec)*1000000+(eTime.tv_usec-sTime.tv_usec); //exeTime 单位是微秒
    cout<<"PlusTime is :   "<<PlusTime<<endl;
    cout<<"AppendTime is : "<<AppendTime<<endl;
    cout<<"SsTime is :     "<<SsTime<<endl;
    cout<<"SprintfTime is :"<<SprintfTime<<endl;
    if(ss==sprintf && append==plus && ss==plus)
    {
        cout<<"They are same"<<endl;
    }
    else
    {
        cout<<"Different!"<<endl;
        cout<<"Sprintf: "<<sprintf<<endl;
        cout<<"ss:        "<<ss<<endl;
        cout<<"Plus:     "<<plus<<endl;
        cout<<"Append:"<<append<<endl;
    }
}