c++ - 在命名空间中定义结构的成员

Question

我正在使用 NeHe 关于 FreeType 和 OpenGL 的教程，但我在定义命名空间 freetype 中的结构 font_data。当我在命名空间中定义其成员时，它不会将 font_data 识别为结构。

CE_Text.h:

#ifndef CE_TEXT
#define CE_TEXT

#include <Common/Headers.h>

/////////////////// MAJOR CREDIT TO NeHe FOR HIS TUTORIAL ON FREETPYE ///////////////////

///Wrap everything in a namespace, that we can use common
///function names like "print" without worrying about
///overlapping with anyone else's code.
namespace freetype {

    //Inside of this namespace, give ourselves the ability
    //to write just "vector" instead of "std::vector"
    using std::vector;

    //Ditto for string.
    using std::string;

    //This holds all of the information related to any
    //freetype font that we want to create.  
    struct font_data{
        float h;            ///< Holds the height of the font.
        GLuint * textures;  ///< Holds the texture id's 
        GLuint list_base;   ///< Holds the first display list id

        //The init function will create a font of
        //of the height h from the file fname.
        void init(const char * fname, unsigned int h);

        //Free all the resources assosiated with the font.
        void clean();
    };

    //The flagship function of the library - this thing will print
    //out text at window coordinates x,y, using the font ft_font.
    //The current modelview matrix will also be applied to the text. 
    void print(const font_data &ft_font, float x, float y, const char *fmt) ;

}

#endif

CE_Text.cpp(我的问题是 void font_data::init):

#include <Common/Headers.h>

using namespace freetype;

namespace freetype {

    ///This function gets the first power of 2 >= the
    ///int that we pass it.
    inline int next_p2 ( int a )
    {
        int rval=1;
        while(rval<a) rval<<=1;
        return rval;
    }

    ///Create a display list coresponding to the give character.
    void make_dlist ( FT_Face face, char ch, GLuint list_base, GLuint * tex_base ) {

        //The first thing we do is get FreeType to render our character
        //into a bitmap.  This actually requires a couple of FreeType commands:

        //Load the Glyph for our character.
        if(FT_Load_Glyph( face, FT_Get_Char_Index( face, ch ), FT_LOAD_DEFAULT ))
            throw std::runtime_error("FT_Load_Glyph failed");

        //Move the face's glyph into a Glyph object.
        FT_Glyph glyph;
        if(FT_Get_Glyph( face->glyph, &glyph ))
            throw std::runtime_error("FT_Get_Glyph failed");

        //Convert the glyph to a bitmap.
        FT_Glyph_To_Bitmap( &glyph, ft_render_mode_normal, 0, 1 );
        FT_BitmapGlyph bitmap_glyph = (FT_BitmapGlyph)glyph;

        //This reference will make accessing the bitmap easier
        FT_Bitmap& bitmap=bitmap_glyph->bitmap;

        //Use our helper function to get the widths of
        //the bitmap data that we will need in order to create
        //our texture.
        int width = next_p2( bitmap.width );
        int height = next_p2( bitmap.rows );

        //Allocate memory for the texture data.
        GLubyte* expanded_data = new GLubyte[ 2 * width * height];

        //Here we fill in the data for the expanded bitmap.
        //Notice that we are using two channel bitmap (one for
        //luminocity and one for alpha), but we assign
        //both luminocity and alpha to the value that we
        //find in the FreeType bitmap. 
        //We use the ?: operator so that value which we use
        //will be 0 if we are in the padding zone, and whatever
        //is the the Freetype bitmap otherwise.
        for(int j=0; j <height;j++) {
            for(int i=0; i < width; i++){
                expanded_data[2*(i+j*width)]= expanded_data[2*(i+j*width)+1] = 
                    (i>=bitmap.width || j>=bitmap.rows) ?
                    0 : bitmap.buffer[i + bitmap.width*j];
            }
        }


        //Now we just setup some texture paramaters.
        glBindTexture( GL_TEXTURE_2D, tex_base[ch]);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);

        //Here we actually create the texture itself, notice
        //that we are using GL_LUMINANCE_ALPHA to indicate that
        //we are using 2 channel data.
        glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, width, height,
              0, GL_LUMINANCE_ALPHA, GL_UNSIGNED_BYTE, expanded_data );

        //With the texture created, we don't need to expanded data anymore
        delete [] expanded_data;

        //So now we can create the display list
        glNewList(list_base+ch,GL_COMPILE);

        glBindTexture(GL_TEXTURE_2D,tex_base[ch]);

        glPushMatrix();

        //first we need to move over a little so that
        //the character has the right amount of space
        //between it and the one before it.
        glTranslatef(bitmap_glyph->left,0,0);

        //Now we move down a little in the case that the
        //bitmap extends past the bottom of the line 
        //(this is only true for characters like 'g' or 'y'.
        glTranslatef(0,bitmap_glyph->top-bitmap.rows,0);

        //Now we need to account for the fact that many of
        //our textures are filled with empty padding space.
        //We figure what portion of the texture is used by 
        //the actual character and store that information in 
        //the x and y variables, then when we draw the
        //quad, we will only reference the parts of the texture
        //that we contain the character itself.
        float   x=(float)bitmap.width / (float)width,
                y=(float)bitmap.rows / (float)height;

        //Here we draw the texturemaped quads.
        //The bitmap that we got from FreeType was not 
        //oriented quite like we would like it to be,
        //so we need to link the texture to the quad
        //so that the result will be properly aligned.
        glBegin(GL_QUADS);
        glTexCoord2d(0,0); glVertex2f(0,bitmap.rows);
        glTexCoord2d(0,y); glVertex2f(0,0);
        glTexCoord2d(x,y); glVertex2f(bitmap.width,0);
        glTexCoord2d(x,0); glVertex2f(bitmap.width,bitmap.rows);
        glEnd();
        glPopMatrix();
        glTranslatef(face->glyph->advance.x >> 6 ,0,0);


        //increment the raster position as if we were a bitmap font.
        //(only needed if you want to calculate text length)
        //glBitmap(0,0,0,0,face->glyph->advance.x >> 6,0,NULL);

        //Finnish the display list
        glEndList();
    }


    void font_data::init(const char * fname, unsigned int h) {
        //Allocate some memory to store the texture ids.
        textures = new GLuint[128];

        this->h=h;

        //Create and initilize a freetype font library.
        FT_Library library;
        if (FT_Init_FreeType( &library )) 
            throw std::runtime_error("FT_Init_FreeType failed");

        //The object in which Freetype holds information on a given
        //font is called a "face".
        FT_Face face;

        //This is where we load in the font information from the file.
        //Of all the places where the code might die, this is the most likely,
        //as FT_New_Face will die if the font file does not exist or is somehow broken.
        if (FT_New_Face( library, fname, 0, &face )) 
            throw std::runtime_error("FT_New_Face failed (there is probably a problem with your font file)");

        //For some twisted reason, Freetype measures font size
        //in terms of 1/64ths of pixels.  Thus, to make a font
        //h pixels high, we need to request a size of h*64.
        //(h << 6 is just a prettier way of writting h*64)
        FT_Set_Char_Size( face, h << 6, h << 6, 96, 96);

        //Here we ask opengl to allocate resources for
        //all the textures and displays lists which we
        //are about to create.  
        list_base=glGenLists(128);
        glGenTextures( 128, textures );

        //This is where we actually create each of the fonts display lists.
        for(unsigned char i=0;i<128;i++)
            make_dlist(face,i,list_base,textures);

        //We don't need the face information now that the display
        //lists have been created, so we free the assosiated resources.
        FT_Done_Face(face);

        //Ditto for the library.
        FT_Done_FreeType(library);
    }

    void font_data::clean() {
        glDeleteLists(list_base,128);
        glDeleteTextures(128,textures);
        delete [] textures;
    }

    /// A fairly straight forward function that pushes
    /// a projection matrix that will make object world 
    /// coordinates identical to window coordinates.
    inline void pushScreenCoordinateMatrix() {
        glPushAttrib(GL_TRANSFORM_BIT);
        GLint   viewport[4];
        glGetIntegerv(GL_VIEWPORT, viewport);
        glMatrixMode(GL_PROJECTION);
        glPushMatrix();
        glLoadIdentity();
        gluOrtho2D(viewport[0],viewport[2],viewport[1],viewport[3]);
        glPopAttrib();
    }

    /// Pops the projection matrix without changing the current
    /// MatrixMode.
    inline void pop_projection_matrix() {
        glPushAttrib(GL_TRANSFORM_BIT);
        glMatrixMode(GL_PROJECTION);
        glPopMatrix();
        glPopAttrib();
    }

    ///Much like Nehe's glPrint function, but modified to work
    ///with freetype fonts.
    void print(const font_data &ft_font, float x, float y, const char *fmt, ...)  {

        // We want a coordinate system where things coresponding to window pixels.
        pushScreenCoordinateMatrix();                   

        GLuint font=ft_font.list_base;
        float h=ft_font.h/.63f;                     //We make the height about 1.5* that of

        char        text[256];                              // Holds Our String
        va_list     ap;                                     // Pointer To List Of Arguments

        if (fmt == NULL)                                    // If There's No Text
            *text=0;                                            // Do Nothing

        else {
        va_start(ap, fmt);                                  // Parses The String For Variables
            vsprintf(text, fmt, ap);                        // And Converts Symbols To Actual Numbers
        va_end(ap);                                         // Results Are Stored In Text
        }


        //Here is some code to split the text that we have been
        //given into a set of lines.  
        //This could be made much neater by using
        //a regular expression library such as the one avliable from
        //boost.org (I've only done it out by hand to avoid complicating
        //this tutorial with unnecessary library dependencies).
        const char *start_line=text;
        vector<string> lines;
        for(const char *c=text;*c;c++) {
            if(*c=='\n') {
                string line;
                for(const char *n=start_line;n<c;n++) line.append(1,*n);
                lines.push_back(line);
                start_line=c+1;
            }
        }
        if(start_line) {
            string line;
            for(const char *n=start_line;n<c;n++) line.append(1,*n);
            lines.push_back(line);
        }

        glPushAttrib(GL_LIST_BIT | GL_CURRENT_BIT  | GL_ENABLE_BIT | GL_TRANSFORM_BIT); 
        glMatrixMode(GL_MODELVIEW);
        glDisable(GL_LIGHTING);
        glEnable(GL_TEXTURE_2D);
        glDisable(GL_DEPTH_TEST);
        glEnable(GL_BLEND);
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);  

        glListBase(font);

        float modelview_matrix[16]; 
        glGetFloatv(GL_MODELVIEW_MATRIX, modelview_matrix);

        //This is where the text display actually happens.
        //For each line of text we reset the modelview matrix
        //so that the line's text will start in the correct position.
        //Notice that we need to reset the matrix, rather than just translating
        //down by h. This is because when each character is
        //draw it modifies the current matrix so that the next character
        //will be drawn immediatly after it.  
        for(int i=0;i<lines.size();i++) {


            glPushMatrix();
            glLoadIdentity();
            glTranslatef(x,y-h*i,0);
            glMultMatrixf(modelview_matrix);

        //  The commented out raster position stuff can be useful if you need to
        //  know the length of the text that you are creating.
        //  If you decide to use it make sure to also uncomment the glBitmap command
        //  in make_dlist().
        //  glRasterPos2f(0,0);
            glCallLists(lines[i].length(), GL_UNSIGNED_BYTE, lines[i].c_str());
        //  float rpos[4];
        //  glGetFloatv(GL_CURRENT_RASTER_POSITION ,rpos);
        //  float len=x-rpos[0];

            glPopMatrix();



        }


        glPopAttrib();      

        pop_projection_matrix();
    }

}

Answer 1

struct freetype::font_data{

应该是

struct font_data{

font_data 在命名空间 freetype 中的事实已经被周围的 namespace freetype { } 覆盖。

所以，事实上，在您的代码中，您从未声明任何 freetype::font_data 类型是 true!这就好像您试图声明一个 freetype::freetype::font_data 类型。

---

这类似于你不写的方式:

struct T
{
    void T::foo();
};

而是:

struct T
{
    void foo();
};